Custom Power Functions plug design
I eventually made my design available for download over at Shapeways with a CC BY-NC-SA license (login is required in order to download the STL file).

I tested it at my local maker space in PLA on a µDelta 3D printer.

For the electrical contacts, I tried with pins scavenged from a DIP IC socket like Cyril suggested but the pins were a bit fickle and easily broke. I ended up cutting a sheet of conductive metal I had handy (iron alloy?) in thin bands with a paper cutter, folding the bands in two then using that as the base for the electrical contacts. Paper clips should also work.

The holes in the 3D-printed socket might have to be enlarged a bit, especially in the first layers of the print, with a needle heated with a ligher or dipped in acetone.

The resulting socket works nicely. Only time will tell how good they fare in the long run.

Cutting the metal sheet:


Resulting thin metal band:


The metal band is folded in two:


The folded band is inserted through one of the holes, folded again, then through the opposite hole:


Once the electrical contacts are in place, they are secured with hot glue on the back of the socket:


End result:


Snap!