We purchased a foam topper that is 1.5 to 2.0 inches thick. It's the same on both sides - it helped a LOT! I think a regular sized one cost about $50 at Big Lots in Florida.
Don't remember the name of it, though.
Two cautions about it: The packaging said to let it 'relax' and air out for a day or so before using it.
It's flammable and emits bad gases if it catches on fire.
I tend to have allergies, and it didn't seem to bother my breathing any when we were using it.