I don't see why any new CU (as long as the lid isn't part of the sides) couldn't ne flush mounted. It might need a bit of work, but mounting methods that come to mind include :
If it's a close fit, screws from the inside through the sides or top/bottom into the wall.
Screws through the back, may need to pack out.
A couple of bits of angle at sides or top/bottom fixrd to case then screw through that into face of wall. For bonus points, do all 4 sides and neatly mitre the corners.
So, what's the reason the CU can't be replaced and still be flush mounted ?
If it's a close fit, screws from the inside through the sides or top/bottom into the wall.
Screws through the back, may need to pack out.
A couple of bits of angle at sides or top/bottom fixrd to case then screw through that into face of wall. For bonus points, do all 4 sides and neatly mitre the corners.
So, what's the reason the CU can't be replaced and still be flush mounted ?