this is a stock backband in the Rainbow boats, but it's a very effective system that I think can make for an easy retrofit
I like the way the backband really wraps around your lower back giving some good support on more surface area and not just one high-pressure point
it can be made youself with a couple of old cam straps, two screws and a drill.
the trick is to get the webbing loop that leads the rest of the system in the right place. It's probably just easier to use a pop rivet in a rotomolded seat, but a thermoformed seat could use a screw/nut with a couple of washers.