I'm using a plexiglass sheet rubbed on one side with fine sandpaper. It's kind of coarse and not as bright as a good ground glass, but it works pretty well and is very easy to make. You cut the stuff by scoring it three times with an exacto blade and straight edge (ruler), and snapping it along a counter-edge. Dead simple.
Even simpler is to put a translucent Scotch tape on the plexiglas. Renewable when you need and relatively bright.