I ordered the corbin rear seat and backrest combo. It sucks that you cant get the back rest by itself so its $$$. Its not here yet I have been waiting for it for 2 weeks now.
I just found out theat if you arrange it prior you can go to corbin's headquarters, order your seat and hangout in the area while they are making your seat. Sure beats waiting 2 weeks for it, you might want to check it out.