[forchetto]

I think you'll find that a normal nut of the same thread size will fit in the recess occupied by that square nut. It might start to turn as you tighten the screw, so wedge it temporarily with a fine bladed screwdriver to stop it turning while you fully tighten the screw.

What worries me is the amount of vibration the battery must be subjected to so as to loosen the connections. I have never known that to happen. You could try to fit a spring lock washer under the head of the screw, although batteries are not normally fitted with any form of locking washer.

Perhaps you could also try to pad the sides or at least the bottom of the battery with some dense foam, if there's room, to isolate the vibration, otherwise that battery will have a short life.