Although what I said...doesnt take into account if there are any specific rules.

Dont forget though, a box is better than a simple flat plate. A flat plate only mates up with the floor area. A box mates up with the floor, sill etc so is much stronger as has a larger contact area with the car. There isnt much reason for it to be a lot bigger than the actual contact patch of a bolt in cage, so +5mm shouldnt be a problem, and will obviously be a lot bigger than the actual bar diameter.