Marc, from your photo it looks as though your problem occurs because your backrest has no lumbar support. Making something that does have support can be an exercise in frustration. Therefore I would go with a backband as it can be adjusted to support your back where you need it, most likely the small of your back or maybe a little lower.

Because the bungees allowed the backband to go forward I got tired of sitting on my backband upon entering the cockpit, especially during "interesting" situations, I replace the bungees with poly cord. Of course you can no longer pull the band forward to get your cooler behind the seat, but most things go easily enough.

I use a cord on each side of the band from the bottom attachment point through a hole in the lower part of the coaming and then to the top attachment point. Once I have distance and angle set by the length of the cords I put a zip tie where the two halves of each cord exit their respective holes in the coaming to retain the setting.

This also works better than a solid back rest as it allows movement here and there to occasionally give your back a rest with a different angle.

~Bob