One crucial question is what sort of LEDs you're using. If they're diffused ones, you may do better with "plain" (but probably a darker "smoke" color to improve contrast).
If they're "point source", it'll probably look better with a frosted piece: the LEDs will show up as somewhat-diffused spots on the plastic, with good wide-angle visibility.
If you live in/near a city, there's probably a plastics supply house, and they often have scrap bins where they sell the leftovers from custom orders at deep discounts. I used to go there often when I lived near one, and accumulated lots of samples for experimenting with. Usually at a quarter or less the full price. It's a great way to learn how to cut/drill/sand the stuff, and to learn what kind is good for what purposes.