I don't know what you consider cheap but I bought my foam from
Jim & Janice Fix who I see every year at Copperstate.
Back in 1998, foam for our RV-6A was about $200 for both seats which I then glued/cut/formed and had upholstered for another $300.
After 2400+ hours of sitting in those seats, they are just as comfortable as the first time I sat in them, no regrets whatsoever. Here's my
original drawing of how I made our seats using the 3 densities (colored pink/blue/green) of
foam .
I know there are many other fine vendors out there to buy from and many different ways to make seats so good luck in deciding

Rosie
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Paul A. "Rosie" Rosales
Rosamond Skypark (L00), CA
RV-6A, 4200+ hours since 7/4/2000