you need to fit a 47mm galv. back box. some need a double box, and some can be fitted to a single box.
Damn, i went to TLC to grab the 45a outlet connection. But it didn't come with the backing plate. The faceplate doesn't look as though it'll screw onto the backing plate you posted. The screw holes need to be in the centre of the width rather than on the length side of it.