This thing was inspired by https://www.thingiverse.com/thing:1265775, but I couldn't make that one work for an American switch—the backplate was too small to cover the switch box, the holes aren't lined up, and the domed backing (from https://www.thingiverse.com/thing:168418) printed in visually unappealing concentric steps.

So I built this from scratch. I've tried it in a couple of places, and it seems to install tidily. The hollow backing does require supports (without the hollow it doesn't sit flush to the wall), but they came out quite cleanly for me.

Yes

I printed this in white silk PLA, which looks interesting without demanding too much attention.

The backplate is quite thin, and depending on your filament, may be slightly translucent. I backpainted mine with white acrylic to ensure opacity. With more opaque filament that wouldn't be necessary (or scale it on the z axis to make a fatter cover).