The super-cheap answer is that most hardware stores should be able to sell you a small sheet of textured plastic "diffuser" material. Cut to size, mount it over the edges of the sheet metal enclosure thing, and call it good enough for a basement.
The other choice I've been pretty happy with in my garage is the LED shop lights from Costco and elsewhere. They look like the old 4' fluorescent ones, but they don't buzz or flicker, and they have a slimmer enclosure. Still bare bulbs, but meant to be that way. Basic, inexpensive, easy to install.
If you're looking to finish or otherwise improve the decor in this basement, I'm with everybody else. Get a different fixture anywhere, new or used, and replace the whole business.