I'm going to visit a fabricator tomorrow. Depending on what we come up with, I'll make sure he keeps the design to create one on-demand.
I see three ways to do this ...
1) Attach a mount at both sides where the lift bar is attached (there are two holes, one on either side). Then, have an upside down "U" brace to give it more sturdiness at the grab bars (the rear bars). From there, a perforated vertical bar can be placed to handle both the backrest and a rear rack.
2) MY FAVORITE: Attach a simple square bracing to each of the grab bars and do the same thing as above.
3) Create an under-the-seat "lip" which fastens in the described fashion ... open the seat, place, close the seat, go. I think that while this one might be the most aesthetically pleasing, it may very well also be the least sturdy.
Now, I'd be PERFECTLY willing to do this myself but can't figure out where to FIND this shit! LOL
I need parts!