in the middle, out of the way of anything. further in, the better protected. Try to keep them as near to the motors as possible to keep cable length as short as possible.

Our DogXBollox controllers sit next to one of the motors (but were running at 24v). Make sure theyre easy to get at in case of problems.

If you put them above the motors, the cables from the interface will have to be run across any way.