I use a 8" Wheelbarrel inner-tube. I use three velcro elastic straps to secure it to my present seat cushion and it makes it very comfortable.
I buy the inner tube that already has slime in the tube to seal any small punctures. I got my tube at Tractor Supply for $20.00.
I know, I know Wheelbarrel or Wheelbarrow
Wheelbarrel' is not technically a word, but some people say it this way, probably because there is no such thing as a 'barrow.' Just saying!