Mounting a voltmeter and USB socket with 'double D' case configurations. The case width across the flats of the D is 1 inch. Used a step drill to 1". Then using the side of a flat file cut two notchedd flats on to fit the case width. A bastard file was used to open and elongate the holes to fit the case. Case is held in place at the flats. The flats prevent case rotation while tightening the backing nut.