I haven't done it, but I have seen people sand the plexiglass with fine grit sandpaper to give it a frosted look. I think that would be able to defuse the light enough. Also, I'm not at all sure if this would work, but hobby lobby sells thin, maybe 1/16" or 1/32" clear acrylic that might be able to sand and give the frosted look and is cheaper and much thinner than the plexiglass. Just an idea!
Leldon