Nylon straps work well. Just put loops on the enclosure and run a nylon strap through them. The strap can bolt to the trunk floor. They make several forms of latches, the best would probably provide tension as you close it.

 

Lots and lots of velcro.

 

Works, but to have enough to where the box is REALLY not going anywhere while driving/crashing makes it a PIA to get out easily.

If you're driving any sort of SUV/Hatchback/Wagon. You should not use velcro as it can come loose in a wreck and render you paralyzed for the rest of your life. I've had two friends had this happen to them

 

I wish I had taken a picture of my 2007 Accord's sub mount before I was hit head on by a drunk idiot. Anyhow, it involved a piece of angle iron bolted to the floor of the trunk with a two 1/2" bolts. The angle iron had a hole drilled through the middle of it. On the Memphis enclosure I had a solid piece of iron bolted to it with a 1/4" stud (i.e. welded bolt to the iron after the head was chopped off). The 1/4" stud went through the hole in the angle iron and I used a wing nut to secure the enclosure to the angle iron.

Oddly, one would have thought that the 1/4" bolt should have broken in the head-on collision sending the 10" ported enclosure into something in the trunk, but that did not happen. The force of the head on impact caused the two 1/2" bolts to tear through the floor of the trunk sending the enclosure straight into the amp that was mounted on the floor of the trunk behind the rear seat. The angle iron was still connected to the subwoofer box along with parts of the trunk floor connected to the 1/2" bolts.

The enclosure and amp were ruined in that accident and I didn't even bother to take it out of the car.