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...... the straps would attach to the gunnels on each side and are adjustable . possibly a 2nd pair of straps would attach to the seat front at extreme sides .
I could even invision a kayak type soft seat with integrated back support placed on top your regular seat (and of course the strap system) .
Take a look at the headers here on p.net. (across the top of screen) ... look in both canoe and kayak ... click on accesories ... and then start looking at seats , seat bands , etc. . You'll see lots of soft seat type ideas . Google kayak seats on the web. and see similar seat stuff .
Anything other than a hard bar frame that digs into your back . Those stadium type seats are not comfortable to just sit in let alone move around in while paddling ...
Another thing to remember when you have a stiff frame seat back is that it sticks up and should you want to duck under a fallen tree or weave througfh similar ... your seat back might be an obstical that's in the way and snag maybe .
I can think off homemade seat backs that flip up and flip back down . Wouldn't be difficult to design and are held up w/straps that simply hook onto a keeper attached to gunnels . There are other ways than straps to lock them in the upright position too . Maybe a single rod from side to side being the hinge axle . This way you've got a nice padded back rest that's wide . In this design idea your gunnels on each side carry the rod axle just behind the seat .
In any case you want something that is light , detaches from the canoe , and also lays down fast when you want it to .
I could possibly help you with the homemade design idea if you want ... can you do the building of it ??