CLC sells what looks exactly like a fine solution for a backrest for about 30 bucks or so. It's a nylon thing, sewn with padding and straps, that you screw to the inside of the cockpit. But I'm a cheap bastard, so I decided to make my own backrest. I cut out a couple pieces of ply, slathered them with epoxy, and after looking around for a somewhat back-shaped curve I could use as a gluing mold, I noticed my '69 Chevy C20 with it's 16.5" rims. Perfect. So I strapped the glue-lam assembly to the spare tire with ratchet straps, blocks, braces, and lots of clamps and let it cure overnight. The result was a nicely curved backrest piece, very light and very strong.

I had some waterproof padding lying around, so I stuck it on the seat back with contact cement, then attached some nylon strapping and bungee cord to the back so that I could suspend it in the cockpit in an adjustable way.

First I attached some nylon strapping to the boat just behind the hip braces on each side, choosing screws carefully so they wouldn't poke through the hull.  (And now you can see the hip braces - those trapazoidal pieces, which I'll fit with closed cell foam to make a snug fit).

I attached the seat back to the cockpit strapping with some fastex adjustable buckles. I then screwed some little nylon bungee cord holders to the bottomside of the deck just below the coaming. The bungee cord is just to keep the seatback from flopping around when you lean forward.  The whole apparatus is very comfortable, it was easy enough to make, and it came out great, but the question you gotta ask is, "How much is $30 bucks worth to you?" If you're (a) loaded, or (b) not a cheap bastard like I am, it's probably worth the money to just buy the one CLC sells.