+1. When I purchased my trike, I included an optional pad that was uncovered, and meant to be slipped into the space between the mesh you sit on and the ties. However, when I put that huge 9C on as a mid drive, my weight caused the seat to sag down to the motor. By removing the foam insert, the mesh took up the sag, and solved the problem. So next, I just placed the foam on top of the mesh, held on with one strap around everything where the seat bends. I liked the feel better than when the foam was under the seat. I recently picked up a yellow terry bath towel for $5 and just wrapped it around the foam and strapped the whole thing back on. Even better. My wife has offered to sew up a terry pillow case for it, but I'm not sure I need to bother.
I'm going to see if I can find a link to the foam pad I used, and I'll post it below:
http://www.utahtrikes.com/PRODINFO-Terratrike_Seat_Foam.html