You’ll need to get something made or design something… maybe you can drill holes and use standard screws to the bracket - that would be secure I guess.
@benmprojects, I don’t have a T1, but I have watched the Youtube that you had linked.
I would recommend that you ask DrZzs. He normally has an open web cast at least once a week that you can join in on. He is also very good about answering emailed questions.
The problem comes with the 4 plastic clips that attach to the external screwed plate, they need more deep space. If the aperture in the plaster is just to fit the rear block then the panel won’t clip. What I did was just press through those holes with a small screwdriver to make more space for the clips, this is in case the material is plasterboard.
Also there is the panel gap( that shows the video) which is not so bad, but it would be better if it was completely touching the wall. I am still thinking to make a 3D printing frame to fill the gap.
Yes I am. There is also the problem if you use the plaster clip metal brackets that hold the mount screws, the clips that hold into the plaster on the outside press against the sonoff mount bracket, sort of bending it. In one i carved the plastic mount, on another i carved the plasterboard.