During use or when not being used? The later is easy. I have got a local tailor to stitch a simple cover using curtain cloth with elastic on the top and bottom. I just slip it on the TV when I am not watching a program.
If you want a cover when you are using the TV, it is more tricky. The best I can think of is to get a large plastic sheet of the kind used as screen protectors for mobile phones. Cut it to the size of the TV screen and stick it using transparent tape.