Fixing deep back box for shower isolator

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 here:
https://btcloud.bt.com/web/app/share/invite/DP9yF4ef9r
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On Friday, 17 February 2017 13:04:25 UTC, Pete Zahut wrote:

Scrape out some of the eggbox filling and insert a noggin between the plasterboard leaves. Then screw through the sides of the metal box into the noggin.

"Error Opera Browser detected"
How charming of BT Cloud.
Owain
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snipped-for-privacy@gowanhill.com formulated the question :

Hmm, that might work, cheers.
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jim expressed precisely :

That's a standard dry-lining box. I need the lugs to be on the other two surfaces (top and bottom as you look at that photo) but nobody makes them.

Maybe, but not hopeful to be honest.
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Um, your switch has screws top and bottom so you want a box that has screw fixings in the same place. Fit that dry lining box vertically and you have what you need, unless I'm missing something obvious...
Tim
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Tim+ submitted this idea :

Only two 10mm twin and earth cables coming in and going out at the bottom :')
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How much slack is there in the cables? Enough to re-route?
Tim
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Tim+ was thinking very hard :

Sadly not. However, Dave Plowman's suggestion gives me hope of getting it sorted without too much hassle.
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Couldn't you just cut a biggish hole in one of the bottom corners of a dry lining enclosure? Dunno about the "legality" of such a modification.
Tim
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Tim+ formulated the question :

I suppose that may work, might just try it as an experiment as I've got Dave's suggestion to fall back on if it doesn't work, cheers.
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I dislike dryline boxes as you can sometimes see them with the fitting in place.
My method works every time and is totally invisible. And as strong as the plasterboard.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 17/02/2017 16:25, Dave Plowman (News) wrote:

Then you need plasterboard flanges

That looks like the OP's best solution as the wall is not deep enough for the back box and plasterboard flanges.
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Adam

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I've found them rather fiddly to use. Perhaps I just need more practice. ;-)

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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 19/02/2017 11:31, Dave Plowman (News) wrote:

It's better than a dryliner box. I think I have mentioned before that I believe that dry liner boxes are made from the devils spunk.
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Adam

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It's probably me, but anything like that made out of plastic with knockouts or whatever seems to always break in the wrong place. Like JBs with that thin snap out part for the wires where the entire side broke off.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Pete Zahut formulated the question :

Aha, yes, I see what you mean Tim - I'm a bit of a numpty lol Yes, I need something that doesn't exist ( a dry-lining box with lugs at top and sides but only a screw fixing at the bottom, to allow for the entry/exit of the cables at the bottom), which is obviously why they went with the metal back box instead.
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Phil L used his keyboard to write :

Maybe - as a last option. Wifey doesn't want it sticking out of the wall but it may have to stick out a bit if we can't do anything else, thanks though.
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Remove the box, and fit a couple of wood battens either side of the opening (1" x 1" will be more than adequate) to the back of the plasterboard, their edges flush with the opening. Screw them in place using screws through the plasterboard and make good the heads - non rusting screws being best.
Then screw the box to the battens using small countersunk head screws that won't foul on the fitting in any way. You may well have to drill holes in the box sides for this.
This method provides an extremely strong fixing for any steel backbox into plasterboard.
If the plasterboard were tiled or well decorated, you might get away with glueing the battens to the back of it and clamping until the glue sets. If you're lucky. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) laid this down on his screen :

Excellent, cheers Dave
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Used to do this for isolating shaver sockets into plasterboard walls, as it took some years before deep enough plasterboard back boxes appered.

Another way (although more suited to a wall which is too crumbly to affix a back box) is to fill the hole with plaster bonding coat, and then push the box back into it. Leave plaster to set (wipe clear of any lugs before it sets). This won't budge. A bag of old plaster is ideal for this as it will set very quickly.
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Andrew Gabriel
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