I used to use adhesive rubber protectors. And those are fine and all but the best rubber protectors used to be those overhead projector sheets. You just cut those to size. Now because those things are getting harder & harder to find, ultimately what you're looking for is a flat, somewhat thick piece of clear plastic.
Well if you've never been to a costco or any sort of bakery and have purchased something from there you have plenty of this. You've just been throwing them away. Don't. You have free rubber protectors right there with just a minimal amount of work. Cut those to size, Boom. Done. You have rubber protectors for stuff you were purchasing anyways.
If it's completely clear? Great. If there's a sticker on it? Pull it off. Pro tip: If the sticker is being a pain to come off, WD40 is great for getting that glue residue off. Just spray it, let it sit for a second. Scrap it off with some sort of plastic card. Wash it afterwards under a sink to get that WD40 oily residue off it.