I have followed your project with interest, Spyd3r, since I plan to attempt something similar this winter. For my project, I went to Bent Bike, a nearby motorcycle wrecker, and bought a used Hondaline rack and backrest unit for a few bucks. As you can see from the photos, the backrest adjusts forth and back. The challenge, as you have found, will be mounting it securely, without interfering with the seat opening.
I like the look of your project and admire your DIY spirit.Best of luck with it!
I like the look of your project and admire your DIY spirit.Best of luck with it!