Drill a hole, grommet, silicone, and stainless hardware.

 

Rigging for an accident where crash loads can be in excess of 9 g's and still be easily removable with hidden mounting hardware... you're not asking for much! LOL [emoji38]

Any magnet that could hold down 500# would have to be mounted in such a manner that the crash loading would be pulling directly away from the magnet as magnets don't hold nearly their rated capacity in shear loading.

The only removable sub box I've ever built that was crash worthy used road box clasps (2 on each long side and one on each end) to clamp the box to the floor but it certainly wasn't invisible.

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That's going to be tricky. You could secure the box to a piece of wood and then secure that piece of wood to the trunk floor. Then hide the mounting hardware behind a beauty panel. Another option would be to rebuild the box to get the same effect. You would have an empty space on either side of the enclosure (or the front, or the back, or anywhere). Like building a false wall into the enclosure. You could then hide the mounts behind panels that just pop off -- perhaps held on by magnets. You could even use some type of quick release clips or clamps.

 

EDIT: Just noticed this at the end of your post...

Straps are not an option.

I used a nylon buckle strap to secure my enclosure. I can easily remove my sub by reaching behind the box and releasing the buckle. It can sometimes be tricky to get it to line up and snap together because I have mine set pretty tight. It's easier to access the buckle through the ski pass-thru. I also sandwiched a piece of steel between the nylon strap to help with strength fastening to the enclosure and part of the trunk frame.

 

I would make a couple flat "L" shaped brackets that bolt to the floor and protrude up through the carpet between the enclosure and the seat.
Couple bolts/screws into the rear of the enclosure and you're done.
Fold the seats down, zip out a couple screws/bolts, and it's out.

I've also used exhaust strapping bolted to the rear deck, and the rear of the enclosure.

 

I use french cleats in my truck.

 

It really doesn't matter how you anchor it if you're in a rear end collision, nothings going to hold it.. that being said, you're really just looking for a way to keep it from sliding around in the trunk. There is no way in hell I would ever sink lag bolts through the bottom of my trunk. Never. But to keep it sliding around in the trunk I use high quality hook and loop velcro. Not only does it completely keep the box from sliding around.. it's still easily removable in the event you need/want to pull it out for whatever reason.

 

I know it's a tall order to keep the box secure in an accident but I want to minimize the risk of it breaking free if I can.

The French cleat idea is great. But after looking at it, would make the box sit up high as I would have to build a platform under it.
So this is what I came up with:

1/4-20 Rivnuts installed into the floor of the trunk (checking for clearance first), laser cut 1/8" steel plates with keyholes on the ends of the box screwed to the bottom at 4 places with #10 course threaded screws. Put the sub in place and bolt down with 1/4-20" bolts. To remove, loosen the 1/4-20 bolts, drop the rear seat, slide the box toward the seat so the bolt heads are in the large clearance area of the keyhole, lift off and remove. I am planning on getting a new trunk carpet that will help reduce some road noise and go around the subwoofer to cover the hardware. Thanks for the ideas everyone!

 

Are you just trying to keep it from sliding/tipping over, or are you worried about it flying around in the event of a crash? To me, if it's in the trunk it's not going to make its way into the cabin, and if the vehicle is in that bad of a crash then a subwoofer getting tore up will be the least of your concerns. Now if it was in a hatchback or SUV it's a different story. I can see why you'd want to keep it from moving around under normal driving conditions but seems like that would be much simpler.

 

Which is exactly why I use hook and loop velcro...

 

I have no idea why my post showed up in Bold. I did nothing different than usual.

 

maybe you can make a metal structure/frame around your box and attach clamps to your chassis that clamp the structure/frame
The clamps will allow easy removal.

 

 

or you can go to a junk yard and get some seat belts and use those to secure the sub box

 

bolt the enclosure down to the cars metal. lay some neoprene between the metal and enclosure to decouple it from the car and to prevent unwanted buzzes/resonances and similar noise that accour when two hard surfaces lay one against another.

 

Something like snap-loc trac. You can cut the track smaller (there's also more discrete versions). One set on either end of the back of the box and Anderson Connectors for the wires. You could just put the seat down to release the connector. Wouldn't see a thing. If you did a bolt through with washers in the box and vehicle body it would hold in an accident unless the box ripped apart or you went Tiger Woods style.

 

Yeah, those things look strong. I like that idea...
Tiger Woods 🤣

 

So I ended up using something I found by accident browsing online. A company in Australia makes these plastic locking connectors called Striplox. Striplox Clip 50 Connector
The small ones that I used can support around 200lbs each. I used a total of 4, fastened to the trunk floor where the sheet metal was flat and available under my sub box. Checked under the car for clearances. After a lot of planning, measuring, measuring and more measuring, I came up with this:

Striplox bottoms "receivers" shown here with PVC risers that I made so the box would clear the carpet and the wiring. The Striplox have a locking tab for permanent installation. They can be unlocked with a thin screwdriver blade, but under the sub box I would not be able to get in there to press the tabs so I cut them off so the sub could be removed and re-installed as necessary.

Removed the sound deadening to ensure the blocks would be flat against the sheet metal. Used the CAD template to center punch, drill and install M4 Riv-nuts. Then bolted the spacer/Striplox stacks to the floor with M4 flat head Grade 10.9 machine screws for added strength.

The bottom Striplox "receivers" openings face the rear seats. Carpet was cut out around the Striplox. The removable trunk carpet panel over the spare tire well had to be slotted so the panel could be removed and installed without removing the sub.

Scored the back of the plastic trunk floor with a utility knife so it would hinge just in front of the sub box for ease of access to the amp and DSP and wiring. Making the floor hinge was also the only way to allow the panel to taken out or put into the trunk.

Here the bottom of the HO110 sub box shown with the Striplox "cleats" that mate up with the Striplox "receivers". I used a template from the CAD model so all the parts would line up and drilled pilot holes so the screws wouldn't go off mark. A total of 12 coarse thread #8 x 3/4" MDF screws.

The seats are folded down, the box is placed in the trunk and lined up on the Striplox.

With a hefty tug toward the back of the car, the box locks into position.

Sliding the hinging trunk floor forward under the sub box.

Trunk floor in position and sub connected.

Folding up the rear seats locks the sub in preventing it from sliding forward should it even try to break free from the Striplox. Attempting to move the sub forcibly rocks the whole car. It's not going anywhere.

I am really pleased how this turned out. Taking some time and exploring all options has paid off. Now I need to carpet or paint the amp rack in the spare tire well.