Light switch blocking cover

We wanted to prevent our preschooler from using the heated fan switch in her bathroom (to prevent its being left on). This should do the job.

This design is inspired by [this one](https://www.thingiverse.com/thing:6688), but is enclosed on both sides. No supports needed. When I printed it, the overhangs sagged inside the switch cover, but the outside was still good, and there was enough tolerance that the sag wasn't a problem.