I went to an art supply store yesterday and picked up some "acrylic sealer" in a spray can for about $5. It comes in glossy or matte. Last night I test strayed in on some shiny piece of plastic lying around the house; it goes on evenly and it definitely seems to take the shine away. Not sure how it will hold up with wear and tear.

The same art store also carries rolls of similar sticky peel-away, matte finish plastic. I'll pick that up for $10 if the spray doesn't work out.
--Andrew