I've just had this same problem, I bought a standard wall switch blanking plate (cost about a quid) and then carefully drilled & filed a hole in it to suit my switch, took around 2 hours since the material the blank plate is made of is rather easy to chip or shatter.
I then just superglued the switch to the cover and packed behind the switch to the back of the box with plastic so I can't push the switch into the box and it takes the load off the glue.
So far 2 days and it's fine.