I've seen the "hardshell seat" on sale for $60. However, I didn't buy one so I dunno how long you'd have to wait to get one at that price. Because I have a contractor's saw (small portable tablesaw), I made a seatback out of quarter-inch plywood slats (7/8 in. wide) screwed onto wooden ribs at right angles. The three wooden ribs are made of 2x4 cut with a bandsaw in order to get the lumbar curve, and laminated with fiberglass to make them strong. I did this so that the seat back exactly fits my back, and equally supports all parts of it, from my shoulders to my tailbone I laminated it with camping foam from Walmart. The whole seatback weighs about 3#.
If you get a seatback, I'd recommend you get one shaped to exactly fit your back. So when you lean up against it, you are comfortable, and there's no more pressure on your shoulder blades than on the small of your back. At least, if you want to be comfortable riding it for a long time. At the moment, I have a simple seatback that only supports the middle of my back. But I can only pedal for about three hours on it, then I have to dismount because my back hurts too much. That's why I built the larger seatback with wood, fiberglass, and foam.
You can go to a chiropractor's office to get a lumbar cushion. This is a cushion you can put on your car seat to support the small of your back. Maybe you could use this and a board and this cushion to construct a curve that will exactly fit your back. Then you can lay fiberglass onto the cushion to cure, thereby making a lightweight seat. Or you could make a seatback from a two straight pieces of plywood and velcro the cushion onto the junction to support your back uniformly, if you're not so picky about weight.
I had my wooden ribs cut at a cabinet shop because I don't have a bandsaw. Then I leaned up against them to see if they were comfortable. After some more cutting with the saw and smoothing with a Stanley surform wood-shaping tool, they fit my back perfectly.
P.S. You could make your own hardshell seat out of 3mm plywood or sheet aluminum or fiberglass.
Last edited by LWB_guy; 07-31-09 at 08:26 AM.