We live in a 1960s semi-detached dormer bungalow, the internal walls of
which seem to be made of something like an egg-box construction.

There's a 45A double pole switch for the shower mounted outside the
bathroom but whoever fitted it should have used a dry-lining box rather
than a metal back box. Presumably the reason they didn't, is that there
are no dry-lining boxes that have the lugs in the correct orientation
for this situation.

The metal back box that's there at the moment is a very deep one (don't
know if they're 45 or 50mm deep) but we can't get away with anything
shallower because of the two 10mm twin+earth cables and the depth of
the switch.

The problem is, the box is actually touching the plasterboard (or
whatever it is) at the other side of the wall, so there's no depth to
be able to plug and screw and fixings - which is why it needs a
dry-lining box that no one makes!

I've tried gluing the back box in but as soon as there is any movement
of the cables, which there obviously is when connecting the switch, the
glue can't hold and the box moves about again.

Any great ideas what to do? Photo he

https://btcloud.bt.com/web/app/share/invite/DP9yF4ef9r