For what it's worth, this may give someone an idea, considering the reasonable prices in Mihalis' link. I bought a used rear seat here on the forum to modify so I could take the cover off and cut away almost all the foam rubber. When I stapled the cover back on (no problem with the soft plastic material the base is made of), it looked just like it did when the foam was still there. The material just stretches over the gap. Of course, I've still got my good leather-covered rear seat for the rare (for me) times I have a pillion.
Why would I do such a thing?
When I camp, I strap a large, well-stuffed duffel on the back seat using Rok Straps. The long Rok Straps
barely made it around the bag when it was loaded. Lowering the seat made tying the bag down quite a bit easier. If it also lowers the center of gravity by a little, that can't hurt either, right?
The job was easy; I did it with my Ginsu knife.
I've given (fairly short) rides with the doctored seat on and I'm told it's not too bad, comfort-wise.