Very good question and something I've been thinking about. There are slots in the motor bracket which will allow me to tension the belt slightly by moving the motor sideways, I will probably have to widen the slot I cut in the top though. I tightened everything down slowly over a few days and it seems okay but I have a feeling the belt is a bit sloppier this evening than it was yesterday so I might have to come up with something else.
Open to suggestions as always!