According to Hagrinas Mivali :

I made a rather crude drawing of what I'm looking for:

http://pages.sbcglobal.net/hagrinas/...witchcover.jpg

If anybody knows what they are called or where I can get them, I'd
appreciate it.

Ask at a real electrical supply outlet about safety locks
or guards for switches. With a description of what you're
trying to do, I'm sure they could help you.

In a pinch, I suspect you could get a heavy duty cable/conduit/plumbing
mounting bracket to simulate your drawing.

Basically, a metal U, with two holes in the end legs spaced
properly to fit over the cover plate mounting screws. You can
get them in PVC too.

Electrical suppliers will have those in with the electrical conduit.

Real cheap. A few bucks.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.