This is an edited version of the awesome design from PropsFactory: http://www.thingiverse.com/thing:25081 that is for OpenBeam 15mm.
The rail on this is 100mm long, 5mm wide at the base and 3mm tall. The holes for M3 screws are pretty snug also.
When I just scaled his version it decreased the actual rail size signficatly so I just cut the actual rail from the 2020 and put it on a bracket (?) that fits for 15mm sized beam. I also have 625 bearings that I will try to adapt to his V-wheel design as well.
This thing was made with Tinkercad. Edit it online https://tinkercad.com/things/bNKsIR2Uw3m
Mount to your Openbeam or 15mm extrusion