Railcore II Belt Holder for aluminum carriage for square nuts

I made a modification to the belt holder assembly for the railcore to replace the use of heat insert nuts. Now there's a slot for a square nut to slide in to create threads for tensioning the belts. Since the threads are much closer to the screw, it is much easier to tension the belts since the screw engages earlier. Also there's no risk of melted plastic filling the threads.

There's two sizes, one for a thin m3 square nut, and one for a thick m3 square nut. The thin nuts are DIN562 and measu