I made my own for a BikeE seat a long time ago. Bought the cover material from Jo-Anne fabrics. It was just stretchy material in black and the edges are shock cord to hold it in place on the Rans seat. You can get the original foam from Rans
MPST0021B - SEAT BOTTOM FOAM (RECUMBENTS) Foam and cover together costs $52 but is a thinner version of the original
Seats
Source of information:
https://www.bentrideronline.com/mess...d.php?t=110991