I find this the best method also. ^^^^^^^Exactly this. The plaster to the sides of the cut out is not supported, so very weak. It can be reinforced though, using timber battens, without too much trouble:
- Cut the hole out of the lath and plaster for the backbox with multitool (we're gonna use a steel BB).
- Cut a couple of timber battens, something about 2" X 1" should do. They're going to go inside the wall, vertically, to either side of the hole, and need to be long enough to extend beyond a couple of un-cut laths, both top and bottom. About 150 - 200mm long is about right.
- Reach inside the hole, and carefully pull away the plaster 'keys' from either side of the hole where the battens are to go. We want the battens to press right up against the back of the laths without being fouled by any bits of plaster.
- Now the fiddly bit.
- Liberally coat the face of one batten with PVA, water based gripfill, or similar. Maneuver the batten through the hole and into position vertically to the side of the hole, glue side pressed up against the back of the laths. Hold in place, and drive a screw through the plaster, through an un-cut lath, and into the top of the batten. Repeat for the bottom of the batten. You can put screws through the cut laths into the batten too if you like.
- That side is now reinforced, and will be about as strong as you can hope for in a lath and plaster wall. Do the same for the other side of the hole with the other batten. You can now fix the backbox in place by screwing through the sides into the battens.
- Filler over the screw holes and you're done.
If possible though I build a "saddle" out of ply wood that you can fit through the cut out and twist into position.
The saddle is made of 2 stirps of ply as the vertical battens then one strip screwed to the back as the horizontal batten. I pre drill the fixing holes so not to split the lath's and then pull the saddle tight up against the inner wall whilst inserting the fixing screws. The back box can then be fixed in place, packed out if required to the depth needed. This method works best on double socket outlets as the saddle slides through the wider gap then twisted into position.