I contacted light wave and they said they neither had any ilford 500 stuff in stock, nor were they willing to custom manufacture a 4x5 mixing box for me.
So I'm still looking!
There is no need for a mounting plate it fits the hole in the Ilford box.
The two side holes are 32mmH x 25mmW and 58mm from the bottom edge, in the centre
A couple of dimple stop the box falling through the hole, a few self tapping screw in the side should fix that problem.
The top is a lid made to be removed from some reason.