Thanks for the suggestions. Using a curtain might be good enough, if the room outside is dark. Maybe I can just arrange it in a semicircle around the door opening. This is not a room where anyone goes, to get laundry or so, but it would be nice if it was possible for the other people in the family to just come in, without ruining things. That way I might even get some company!
If I use rubylith and put the computer behind some sort of black-painted wall. If I find a way using the computer, I can have my notes at hand, play music and browse APUG while waiting for the lith developer to kick in, all at the same time, so it would actually mean a lot.
Problem is I don't know where to find rubylith. Maybe there is a program for OSX that can make the screen dark red? If no one knows here, I will ask the astronomy guys at cloudynights.com. They need to save their night vision.