I have finally managed to get hold of a router to mill the slots for the motors. These should allow me to tension the chain as it becomes slack. Unfortunately one of the motors isn't quite aligned laterally causing the chain to catch occasionally. I should be able to fix by widening the slots by about 0.5mm. Here's a photo of the underside:

Once that's sorted I can finally start testing the balancing code. I'll also need an enclosure to protect all the electronics and battery.