Hadn't seen this thread before, so let me add what I did - I wanted to be able to later replace the shelves if I did any upgrades/replacements/etc. so I added attachment points for the shelves to be screwed onto.
On the left side, where I may (or may not) want to later mount a rudder trim servo, I based it off of Vans' AHRS kit, except I "doubled" it for a wider shelf with a second VA-260B:
The pre-drilled holes on the bottom flange of that part don't line up with the existing rivet holes properly, but they happened to have just the right spacing that I added another hole inbetween each pair of pre-drilled holes, and now it's securely mounted.
For the right side, I added two lower "rails" that I can attach the shelf to using another F-1037 (with added holes):
If anyone does this one, be careful with the order in which you rivet it - it's hard or impossible to buck the bottom skin rivets after the F-1037s are riveted on. I riveted the skin first, then used cherry rivets for these (and yet I needed a 1/4" spacer to squeeze some of them 'cause the cherry squeezer's head was too large to reach them).
The plan is to put avionics on the lower (right) shelf and put any higher-power electronics (servo, shunt, APU port contactor, etc.) on the taller left-side shelf.