What I did was use a SeaEagle inflatable seat, which gives glorious back support due to a kind of pyramid rear shape. A simple L shaped rigid seat will just want to tip over backwards, or rip your anchor points if you don't make wooden rails or the like. A simple strap over the small inflato seat holds it except for a bit rocking left and right.
Best not to use their largest size due to high CG, although maybe that is just my height. It's preferable to never deflate it due to 2 bladders being kind of fussy to align with cover geometry. Don't let SeaEagle charge you a fortune for shipping - they had the seats incorrectly listed as needing an oversize box which you can convince them is not needed.
Thanks for reminding me... now I can perhaps use it on an itrek 9 for more legroom. I can also use the original i12s floppy seat as a footrest on non-mirage sit-down paddleboards. I will lay it on it's back and have the seat vertical.