I would think that having a seat back that supported all the way up to your shoulders would be more important than having the proper lumbar curve. It only needs to be 8-11 inches wide, but it should be 15-18 inches tall (from the top of the seat pad.) Seven inches tall is just too short. The EasyRacers Cobra seat is a good example; it is tall but narrow. There's no lumbar curve, the padding compresses where it needs to. Here's a picture of a Recumboni seat, just for reference.
