Support for void cube track ring

For the track rings for http://www.thingiverse.com/thing:158280 (also based on http://www.thingiverse.com/thing:154112).

I could not get the track rings for the void cube to print, even with several layers of "forced support" in slic3r. I did get one to print side up but it makes for uneven tracks, and I'd prefer oriented flat. The area on the bottoms is just so tiny though.

Cooked this support up in scad, and it works great.