Ok got some 6mm plate and a 44mm holes saw.
Made a wooden template , time it met the steel ?
I got enough plate for 2 tries , first up will be cut out the surplus square areas then drill the big hole.
Not going to be a breeze as when the box is rotated it masks some of the mount holes needed for the plate.
I think it I had more confidence/experience the plate could be welded to the mount ?
However I am going down the bolt route , so first up I need a 44mm hole in the plate and 3 holes drilling to attach the plate to the mount.