This is a case for the Sonoff Basic Wifi switch designed to fit tightly enough (like a Speedo) to allow in-wall light switch installation. I do not know if this is safe or legal, so do it at your own risk! It is definitely safer than just wrapping it in electrical tape like one Youtube video I saw.

There are two versions, thin and normal, but I only use the normal one. The normal one is designed to be rigid and thick enough to protect the board while installed. There is a thickness variable in the scad you could tweak if you want to.

The thin variation will only work with flexible material, as it actually wraps around the PCB and holds tight. Trying to get this top on is probably a lot like trying to get a real Speedo on! I printed it in TPU, and it works, but ultimately I decided that I wanted the protection of the Sonoff over the couple of millimeters saved by the super tight design. The thin version is only like two layers thick, so you need to have good printer TPU settings for it to work.

Hah, I didn't realize how many in-wall solutions were already posted! This approach keeps the light switch looking the exact same as before installation, but does heavily modify the Sonoff itself.

No one in the house even noticed the first install I did!