DIY PVC seat

I'd be tempted to just make it out of aluminum tubing. Bend one long piece to the desired shape, get the next size larger pipe to sleeve the joint, and braze in place. Lightweight and will outlast the kayak.

That's a good idea. I might give it a try. Any advice on bending aluminum tubing?

You could get a pipe / tubing bender, but aluminum is pretty fragile. I've never bent 90 degree turns with aluminum tube, but I'm sure there is a method. Maybe just heat it with a propane torch? I've done that with some success before, it softens up pretty easily, and won't crack then. Could take some practice to get the bends just right though. 

I'd be tempted to just make it out of aluminum tubing. Bend one long piece to the desired shape, get the next size larger pipe to sleeve the joint, and braze in place. Lightweight and will outlast the kayak.

That's a good idea. I might give it a try. Any advice on bending aluminum tubing?

You could get a pipe / tubing bender, but aluminum is pretty fragile. I've never bent 90 degree turns with aluminum tube, but I'm sure there is a method. Maybe just heat it with a propane torch? I've done that with some success before, it softens up pretty easily, and won't crack then. Could take some practice to get the bends just right though.

I wish I could afford this machine...

While on hiatus from working as a carpenter, I worked in and then managed a welding/mfr company in the north bay MANY years ago.

We custom bent a lot of ferrous metals and flat aluminum. We also worked with alum pipe and tubing. We mitered or coped most of it. If we needed a radius, we used fittings or pressed in wooden dowels in the area that needed the bend.

This supported the wall as to limit deformation. Without this backup, the pipe lot tubing would crumple, usually on the inside, and sometimes tear on the outside. Flame heating did not work for us.

But this is old info - hopefully new and much better methods are utilized now!