All of the light fixtures in the finished portion of my basement are pretty much just bare bulbs attached to the joist, and then when they put in the drop ceiling, they cut a hole around the light and put in a piece of frosted glass, framed with wood. Unfortunately, the guest bedroom I'm working on was missing the glass... so I needed a cheap & easy alternative.
I found a large piece of window glazing acrylic in my attic, left behind by a previous tenant. It was broken, but still exactly what I needed: a clear piece of hard plastic, not more than 1/8" thick, and big enough to fit inside the wooden frame. I put some 100 grit paper on my random orbital sander, and tried it out on a test piece. It worked perfectly! After that, I sanded a big piece of the acrylic, cut it to fit in the frame, and voila! Faux frosted glass!
Pictures: Light fixture before & after (with light on and off), and the test piece.