Welcome to our forum.
"Booster" cushions are available which are filled with water to adjust thickness...about $50.SB
You can use small glass marbles inserted into the cushion, just enough to weigh them down then top off with water, it works real well.
Guzz,Marbles! Great idea. I'll be searching for lots of marbles on my lunch break tomorrow. Thanks!Phil
If you find mine, let me know.