You can make your own using closed cell foam, which can be
purchased at an outdoor/camping store.
It's used in canoes.
Or I have 3/4" thick self adhesive seats made.
Only have 2 of those left.
GP Tech makes a seat, it's not very comfortable.
I have not tested, but have one made by Woodcraft.
Those are 1/2".
e-mail me for more info