On Saturday, 12 October 2019 09:56:56 UTC+1, Tricky Dicky wrote:
Do the dimensions of metal faceplates not conform to a standard like
the white plastic versions?

No they don't. Even white plastics can vary a bit.

I would use ordinary metal boxes joined with conduit couplers and male bushes. That will get you even spacing with a small gap between.

Owain