I wonder if anyone has come across a metal accessory box, which is 4 gang wide by 2 gang high? (or similar?) I thought that something like that would be good to bring cables through to the back of a CU mounted over the box. If you see what I mean.
All sorts of things you could use - grid boxes go pretty big, or there are cavity floor boxes - both of these will have plenty of knockouts, or there are any number of metal and plastic (why do you want metal, particularly?) adaptable boxes of all shapes and sizes.
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