I followed/adapted the instructions on Surfoos' website about his AI:
http://www.myadventureisland.com, and built my backrest out of small PVC pipe.
The photo (below) shows my assembled seat, and where it's placed on the Hobie AI. I used pipe insulation to soften the backrest against the hull and cross bar (so there are no stress points) and nothing needs to stick down into a scupper hole (again, not creating any new stress points).

I use one of those safety/floatable seat cushions for added support. They are stiff enough to not fall through the gap in the back of the backrest, and can be removed if I want to lean back even further - and there's always the added safety factor.

The seatback has made it a joy to stay out on the water for hours at a time - with no back problems later. It's pretty easy and inexpensive to make, too. I added a gel-type "fanny pad" to the seat's bottom, for a little extra comfort there, too.
Thanks again, Greg, for the great idea!