Thank you so much for all this info!

I was able to check and the mattress was indeed 1.5/cu ft. I was looking at high density foam on Amazon and found some that says it is 1.8/cu ft and "44lb compression", which I assume is the firmness rating you mentioned for the 1600CH. It sounds like this would be too soft since you mentioned that having 80-100 (my dog is ~80 pounds, if that helps).

Looking at packaging foam instead of upholstery foam is an interesting idea. My only concern with ordering it from a place other than Amazon is that I'd like to get it ASAP to help my dog out and that site doesn't show any delivery time estimate for shipping, but if you think getting the polystyrene is going to be significantly better then I will go with that.

Thank you so much for your help <3