I know this is an older thread, but I have and do, frost plexiglass. The best way to get an even frost, and not "harm" the plexi is this: Buy yourself a can of clear "wood" based acrylic sealer/clear coat and spary one side of it lightly. Wait until it drys, do not soak the plexi with it. Re-aply light coats for a thicker frost. It will take about 10 min per frost. Easy, simple, and when working with plexi, a very fast option. I have noticed that it does work best on flat peices, so if you have heated the plexi for bends, it might not be such an even frost on the bended areas. Hope this helps.