Our local astronomy club just gave out red celophane plastic bags. You could probably use a sheet of that (which looks to be about $15 for a package that you could cut yourself?), and keep it on your nightstand.

I know it's neither as convenient nor as easy to use as a case mod or f.lux, but it's worth a shot.

Someone could probably make it work via a manual mechanism built into a case -- a loop of transparent film through the case, filtered on side of the loop, with some sort of locking mech to pull the moveable filter tight against glass during usage.