Well, I loved Gary's solution, but I had already invested in the RnR shelf. It looked like the aluminum, even the less heavy duty type, would cost me $35, plus $20 or so for detent pins, so I decided to rethink.

The time consuming and clumsy part of putting the shelf on is installing the bolts and butterfly nuts, so what if I just switched to detent pins?

I tried various different size pins, but eventually concluded 5/16 x 1.3 inch was the ideal size. I had to drill out the pre-drilled holes in the handles just a little (#8 metric drill) and the holes in the cable tabs (#6 metric drill). I then fastened the cable tabs using the existing bolts under the shelf.

This is a major improvement, with no fumbling and no risk of losing the bolt or nut! Here is a scan of the parts I ordered from Reid Supply, delivered quickly and efficiently by USPS for $25. And photos of the finished project.