Here are a couple prototype ideas for controller safety clips that can mount to the existing Professor Motor controller mounts/perches, whatever you call them.
First is an open ring, based on what Ary was talking about Saturday, the second is a pair of fingers, slightly angled out. Both are shown here with a mounting plate with 6mm holes on 150mm centers. also shown is a clamping plate.I was working from memory here, but the idea is that these can be mounted on the existing hardware without drilling, and height adjustments can be made , so see where they have the best effect.
I think they are both worth a try to see which direction works best.