In the post mortem of my wife's PolyBot assembly, one of the things she wanted was a hole to put a screw through the servo mount to hold it to the platform.

This thing is the exact same as the previous version, with the addition of an M3 sized mounting hole, and an indent for the matchich hex nut on the 'bottom' side of it.

I considered having more holes for more holding goodness, but I'm still sticking with the minimal fasteners design principle.

1) Print a few of these out
2) Fit standard sized servos into them
3) Drill an M3 sized hole in the shelf
4) Mount them on a 1/4" shelf
5) Put a M3 sized screw through the hole
6) Put an M3 sized hex nut under the bottom, and tighten. You might want to put a washer on the topside.
7) Rejoice!

With the OpenScad file, you can easily change anything from the shelf thickness, to the hole size, so you're not stuck with the M3 sizing.