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I would have written this out for you if I realized someone wanted to know how to do it, sorry!
If you're interested to know how a machinist does it, here's how I cut over 2 dozen lens protectors;
For round protectors
1) go buy the adjustable rotary cutting tool specifically made for cutting holes out of thin sheets. (not a hole saw, but that works too I guess)
2) remove the twist drill from it if it has one, clamp your material securely and cut a circle out of it
3) Use a file to bring it to snap-in size and deburr the edges
4) can use glue to secure it if you're not confident with the snap fit
For odd shape protectors
1) Very accurately trace out your shape
2) Using a bandsaw, or preferably a scroll saw (hacksaw works since most of you don't have either of those), cut the rough shape leaving 1mm around the flat edges, more around the rounds.
3) using fabric softjaws in your vice, clamp the lens and proceed to file it to shape using the accurate trace you made as your guide. Proper filing is an EXTREMELY rare skill, even in industry, as anyone who knows how to use a file will tell you. So don't feel bad if your first lens doesn't look amazing lol
4) Deburr your edges with the file, peel off the protective plastic from both sides of the polycarbonate, and snap fit it to you optic. Usually need glue for odd shape protectors.
If you do need to glue a protector in, use a FLEXIBLE glue, like hot glue. DO NOT use superglue or krazy glue. Firstly, krazy glue has a stupidly low surface tension and it'll get all over the polycarbonate lens. Second, as soon as a BB hits it, it'll break loose.
And most importantly. Above ALL other things. When using a file, it ONLY cuts in ONE direction. So don't be dragging it back and forth with all your body weight on it.
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