The people who put the finishing cement on the interior walls of my house did a spectacularly bad job, and each time I had to replace an electrical switch or wall receptacle, a huge chunk of finishing cement would fall off from around the opening for the electrical box where it is the thinnest.
At first I simply put Selley's No More Gaps around the edge of the wall plate to cover up the gap made by the crumbling finishing cement, (not too bad of a solution) but this made it difficult each and every time I would have to go back inside the box to change something.
I made the below drawing on a piece of 8" x 11" sheet of paper, and it is essentially a pattern to make a trim ring to go around the edge of the box to hide the ugly gap. This way I don't have to spend twenty minutes cutting off the old caulking material, and then having to reply it all over again.
If you decide to use the above jpeg image, make sure that the screw holes are 3 1/4" inches apart when it comes off your printer, or it won't fit on a standard single gang box. You can play around with the inside and outside edges, but the screw holes will have to remain in their current positions, or it won't fit.
Next I cut out my drawing and used it to make the below proto type from the cover of a Sky Flakes crackers box. It took me several attempts to make a ring cover that I was happy with, so if your materials are limited I recommend practicing a bit before you go to make your real one.
Next I used some material from a left over gutter downspout to make the actual trim ring that I planned to install onto the wall. I had a left over box lying around, so I used that to make sure that it would fit before I tried attaching it to the box in the wall.
Once I was sure it would fit, I pulled the switch out from the wall (power off) and was able to slip the trim ring I had just made over and around the switch assembly, like putting a button through a button hole. Below is what the finished product looks like. I know it looks kinda silly, but it does the job, and it seems to add more support to the switch cover on some of my boxes in which a lot of finishing cement has broken off.