The heatsink came off an old motherboard and fits dang near perfectly on the side (or bottom) of the X-motor with a ziptie and a generous dose of thermal compound. The dimensions are roughly 1.5" x 1.5" x 1.0"
The 12v 1.5w fan was from radioshack, also roughly 1.5" x 1.5" so it fit the top of the heatsink very nicely w/ some epoxy.
My projects don't generally require long prints, but upwards of a 2 hour print, even with the metal cover on, the X-motor doesn't get terribly warmer than the Y- or Z- motors. Warm, but certainly "grab-and-hold" touchable.
While you're at it, I would check the temperature of the 5v regulator- it is the 3-legged black device with a metal tab on the "top" of the motherboard. Mine was getting warm even just sitting idle. I've heard of them popping on people. If yours gets hot, you might consider attaching a heatsink there, too- or at least blowing a fan.
I also cut a trace on the solidoodle motherboard, but you could achieve the same goal with less hassle: cut the USB cable in half, and reattach all wires except for the +5v supply (red). This will help remove possible stress on the regulator, without requiring direct modification of your printer- and if you kill the USB cable on accident, it's easy to find a replacement. The only caveat to this mod is you need to power the printer for your PC to recognize it as 'plugged in'.. which is the general use-case, anyway 
If it sounds risky, it's not- you are actually putting your machine back into USB Spec by doing this mod.