I've tried back bands with multiple straps to adjust the angle and they don't work. The only system I've found that's foolproof is to attach a loop near the center of the upper edge of the back band and connect it to the coaming with bungee cord. This pulls the upper edge of the back band to the rear when you're not in the boat. As you slide into the cockpit from the rear deck, it creates a ramp that guides your butt into the seat. When you push back against the back band, the bungee allows it to tip forward so it sits flat against your back. It's really a slick system.

BTW, I agree with your use of a padded, rigid back band. I had been using typical touring back bands which are relatively flexible. Once they break in - or break down - you end up pressing against little more than a narrow nylon strap. Not comfortable. I just installed a whitewater backband from Voyageur in my 'glass boat. It has a nylon strip for stiffness and firmer padding. It is MUCH more comfortable than the floppy nylon back bands. The ultimate may be the Immersion Research Wankle or Reggie back bands. I have a Wankle that I stuck in my boat and it felt great, but needs some modification in order to fit properly. The Reggie would probably be a better fit, since it's a bit smaller. I may give it another shot at a later date.