Glad you liked it!
I have not experienced any trouble with the belt not being parallel. Since the part is a tight fit for the holding block, you might need to sand it down a little more to give it some "wiggle space". Otherwise it may be pushed off-center by jamming it in the block.
If the problem still persists, you might also want to try to reposition the toothed pulley on the motor slightly to make your belt parallel.