I picked up a couple of glass cubes with traditional bulbs in them from a charity shop. I've rigged up a bunch of three-colour LEDs which crossfade to go inside instead of the lights that came with.
The LEDs are clear though, and while the inside of the glass cubes are frosted there's obvious spotting and you can see the separate LED colours when more than one are on at once.
Sandpaper (skuff up the plastic housing), straws (cut off a small piece and cover the housing), ping pong balls (put the LED inside), and styrofoam (shape a small piece and put the LED inside) can all work.
I have personally had pretty good success with sandpaper.
I think wax paper (not sure what you might call it there) does a great job as a pre-diffuser. Cut out a square and crumple it over the LED and place that in your frosted cubes.