What translucent material can I use to cover the bulbs at the top of a medicine cabinet?
You could certainly use frosted glass, though acrylic is easier to work with and lighter weight. You can cut it with a saw, it's effectively "shatterproof," and the edges aren't sharp like glass.



Frosted acrylic sheets are available at many DIY "big box" stores, and online. You may know this acrylic material by the brand name Plexiglas.

Acrylic sheet is also available in a variety of patterns and textures, as shown on one manufacturer's website.

One easy way to go might be to find a replacement diffuser panel for an overhead fluorescent light fixture. and cut it to fit your application.