You are correct, the concept is that you want a little bit of light coming through the trap so the bees want to plug the holes. But even if you just cut inner covers out of window screen you’ll get a lot of propolis on them. Cut the screen to fit the box. Peel it off when it’s well covered and roll it up and rubber band it. Put it in the freezer. When you pull it out, put it in a garbage back and unroll it. The propolis will fly off the screen. It comes off of the plastic better… but either will work. The screen might be free if you have some laying around. If you want to collect it for your use or to sell it, this works well. The propolis will be clean (no wood chips), unlike what you scrape off in the hive. It’s still worth keeping the scrapings from the hive, but it won’t be as clean.
Propolis is a good immune booster and a good antimicrobial for any kind of sore. It makes nice “chewing gum” but may stain your teeth. It has been proven to prevent cavities when used in a tincture as a mouthwash, but again, will stain your teeth… I used to take propolis but haven’t bothered in recent times. I have a good immune system, so I can’t tell that much difference.