Sheep, go down Van Ness south of market, just before Smart & Final is a plastics shop where you can buy yourself sheets of Perspex for next to nothing. Make sure you have the measurements you need though as they’ll cut them up for you at the store.
I’ve actually got a couple sitting in front of my screens, stuck on the borders using sticky backed velco (so i can easily pull em off, clean em etc). They’re serving the dual purpose of keeping the dust off the screens, and of stopping my cats from damaging the screens when they want to attack anything moving on screen, but I could additionally draw on them with marker if I wanted to.