USE AT YOUR OWN RISK!!!
I bought a Shopsmith that was missing a number of parts, one of which was the power coupler that drives the belt sander. I measured the keyed collars (which fortunately DID come with the machine), spent about 1/2 hour in Sketchup to produce the attached STL. The fit is snug, but almost spot-on. My print head was a bit too close to the bed, so the 4 groove side was a bit squished, but a few taps with a mallet got it on. Video of it in action is here: http://youtu.be/h-FRnJkr3hk
This probably won't break away as the OEM part will, so I repeat, USE AT YOUR OWN RISK! I printed mine in ABS, but PLA should work too.