This small opening was there because I wasn’t sure whether I would build up a bit here to mount additional switched into or not:
But not needing any more switches I decided to run with:
And this is how it turned out:
I had to take the door off to drill the holes for the self tappers. This seemed a touch extreme, but I think was the best option. The other options were trying to drive the self tappers into the plastic without pilot holes using a stubby screw driver. This was going to be really hard work.