my seat guy used auto seat resto foam,(dark gray) that could be shaped with an electric knife, then i had him cover it with 2" of some really squishy foam stuff, which compressed when he stretched and secured the custom cover.
my @$$ is happy.
my suggestion would be to buy a few layers of different densities (maybe a 3
" heavy duty layer, and a 2" softer top layer) stack them, and give it the old seat-o-yer-pants test, knowing that the uncovered foam will be compressed when covered.