Thanks for your responses
@SUNRAY. I can follow what you did.
In my situation, I could use your method or alternatively, the galvanised extension boxes as I think it will give me the correct depth. The reason, I am interested in the cut PB box approach is that, I don't have to be too precise with my cut in the panels. As long as the cable in the existing galvanised box can be fed through to the cut PB box, we are good, as I understand it. This means that I don't need to line my cut exactly with the galvanised box (and the extension or PB box that screws into it). Hope that makes sense.
If there is merit in what I am explaining above, the cut PB box may be preferable and I assume that it will need to clamp onto slats on bot ends. Again the above method will give me some flexibility so I can move to make this the case. I just don't know how the lugs will work if I reduce this to 21mm depth. I'd appreciate your thoughts.