I use to live by long wood gardens in PA and they have all their orchids in one area of their green house with frosted white glass. All of them seem to do just fine and there are a lot of different kinds in there. I would think a piece of frosted plexiglas will work just fine. I would just make sure your light doesn't get hot enough to melt any of the plexiglas. Maybe put some space between your light and the plexiglas.