Just helped do an installation of a tankless water heater for a
customer. My part of it was wiring the heater (it's got electric
ignition). It's wired to a switch inside the house. The customer is
concerned that the switch could be mistaken for a light switch and
turned off, as am I, so I looked for some kind of cover for it with no
luck. I was hoping to find something like the little rectangular metal
box covers that one sees on furnace switches; you know, the ones that
flip up to. The only ones I could find, however, have a switch and a
fuse mounted in a duplex box, with the cover over the fuse, not the
switch (I don't need a fuse anyhow).

These little covers would be perfect. Are these available anywhere for
use with switches? I'm thinking either a cover plate that has the safety
cover built into it, or a cover that could be screwed over an exisiting
switch plate.

I did find some interesting switch covers on line. These ones

http://www.epinions.com/content_406023409284

fit over any standard light switch that you want to make sure either
doesn't get turned on or off by mistake. However, they're a bit
expensive, I think, plus they can get lost.

The other alternative that I'm going to use in the meantime are those
little plastic tabs you can screw onto a switch to keep it in one
position or the other. Also not ideal, since you then need a tool to
flip the switch.