I just started a thread a few days ago about this problem. I am going to be using lead sheeting in replacement to MLV because of the thickness. A 1/64th in sheet of lead is directly comparable to 1/8in MLV. I posted a couple pictures comparing thickness of 2lb lead to 1lb MLV as of now since I am still waiting on CCF from Resonix. Will be posting more comparisons there when I can install. Thread is in general.
Also, if ABS sheet is too thick for the panel, you can make fiberglass sheets mounded to your door to do the same thing.

 

I’m about to buy one to use as my work truck, in my opinion for best results in a door location is to have the speaker protrude beyond the door card so don’t be afraid to cut the area and make a small cover from fiberglass or get a regular speaker grill that’s big enough to cover the area that you cut, screw it to the door card from behind, completely put the door card on, tape up a few inches around the ring and use Duramix or kitty hair to blend it to the card, take it off when dry, sand and use body filler to make it nice and smooth then wrap it with matching vinyl or paint it. 90% of spot treatment after installation is to the door card itself, doing this will eliminate that problem.