Trace the lightswitch opening on the tin. Then mark an "X" from corner to corner of the opening. Cut the "X" out with and exacto knife. (Cheap and they come in handy.) Bend the little triangles to the back. You can probably use a hole punch for the screws to go in.

Do not let the inspector see the homemade covers. They are supposed to be listed (by UL) to be used. If you look in the NEC it specifies the minimum gauge required for metal plates and the thickness for non-conductive plates.

brickeyee, do you know offhand the codes for this? I've seen copper switchplate blanks (sold for copper enameling) and was thinking about doing a verdigris patina on them. Does it need to have some kind of non-conductive backing? Plus, they don't have all the configurations I need, I would have to make the rest.

They are listed in the NEC. Your library should have a copy. (Mine lives at work, not at home).
I would doubt the blanks you saw are UL either, but they could be if they are thick enough.
Conductive metal switchplates are allowed, as are plastic of greater thickness. Wood or flammable plates must be lined with metal for fire resistance.

The ones for enamelling are 16 gauge copper. They have to be thick so they won't warp in the kiln. Recently I've seen some artists selling enamelled switchplates at art fairs. I'm more interested in getting the blanks and maybe etching a design into them, then doing the verdigris patina. I'm a part time metalsmith and do a fair amount of etching, so thought it could be nice to bring that into my new kitchen.

Having once gotten a serious shock from the cover plate screw of a box where the hot wire was touching the side of the metal box, I've been leary about conductive cover plates. Since then, when I've used metal cover plates, I've glued a thin rubber backing on and used nylon screws to hold the cover plate. I'm not going to claim it's UL listed, but I feel better about them.

"Having once gotten a serious shock from the cover plate screw of a box where the hot wire was touching the side of the metal box,..."

You have more serious problem than a hot touching the plate. The plate is not grounded, and that means the switch is not grounded either.
Using a makeshift repair to hide a more serious problem is not a very good idea.

brickeyee-
It was in an old house where there was no grounded wiring. I subsequently re-did all the wiring with 12-2 grounded. I was just making the point that I prefer not having anything directly conductive on the exterior of a box...particularly if it's old and ungrounded.