The mat was too expensive (21$ for same sqft coverage). But you are right, it would be a little thicker. The back is 3 layers + a little fiberglass Bondo in between. (the amount of curves given back there, it is ridiculously stiff).

The front is fleece (I learned from this build to not to buy too thick of fleece! But.. it absorbed a crapload of resin and is thick in itself. There are 3 layers in the front, but where the large flat area is on the left, I have added total of 6 layers there due to flex. Will probably add another 2 layers tonight even though it's pretty tough there now.

What I do is place the sub in the box and then slowly turn up music until I hear rattling. Then go back and check the box where is is flexing / rattling. The only place was that large flat space. everything else was good so far. And it was loud enough for neighbors to hear inside their home next door.

There are air pockets. (I filled those with folder fiberglass then poured resin in them.

There are tons of strands on the inside.
I will also be stuffing fiberglass insulation inside that corner to help dissipate the pressure. I should have started over when I farked up the molded piece. But I just keep testing and fixing till it works.

Thanks for the weatherstripping idea. I will be screwing this unit into the plastic liners also to keep it from no sitting perfectly flush (top tends to want to pop out 1/2 inch).