The original holders fit the Traxxas ball ends perfectly (thanks!), but I was too lazy to go get a piece of wood from the hardware store, then measure, cut, mark, drill, etc. Since I already had the OpenBeam for the printer... why not just reuse it. So I made this minor modification, reusing all the hard work for the ball-end holder shape. Number of holders is reconfigurable.
Pretty self-explanatory: thickness should work with 8mm screws (same as for most of the Kossel frame), just slide jigs into extrusion, clip on a rod to adjust length, and tighten down. Then do your gluing.
I glued all rods using the same jig (one at a time) -- but if you are too impatient for that, you could use the copy-pasted version and two (or even six?) parallel beams (untested). If you want to recompile for a different number of holders, you'll also need the STL from the source thing.