Ah, I thought that it was fabric store foam. I've seen fabric store spun polyester that comes in a lot of different densities. With a lightly compresses batting, I'd go for thicker as well. The stuff that I use is pretty dense which allows it to be thinner, see below.
Whenever I experiment with linings, I find it pretty difficult to judge before and after because of the time delay in opening the amp, futzing with the material, and retrying. I feed the same song from a CD into the amp to be consistent. Then when I think that I've got it best, I do playing tests.