I've never had much success with using blisters

I have/had/will have again when the muse strikes, a long term project to build 6mm science fiction scenery from blister packs. I ended up treating the blister as a mold and filling it with plaster, but it wasn't great.
The problem I always run into with trying to layer something on the inside is that any pressure on the outside will cause it to crack, as getting something to stick reliably to the plastic is difficult (as you're finding).
I haven't tried it, and it would be messy, but maybe something like papier mache rather than expanded foam. This would completely fill the inside of the blister, which should make it more resistant to being leaned on. Maybe add lumps of polystyrene in the mix to use as a filler to keep the weight down (although that will depend on the size of the blister)
Curious to see what others suggest - and may try the texture gel idea myself, as I have a few tubs lying about the place