airsix
Contributor
Well, I left that part out because I didn't want to discourage anybody. I used a rotary table on a milling machine. Wait! Don't leave yet!!! You don't need fancy machine tools to do this.
You could do this many different ways that are just as effective but take a little longer. Step one is to mark a circle on the polycarb sheet with a marker and rough-cut the disk out with a saw. It's kind of brittle so support it well or it'll fracture. Once you have an oversized somewhat-round piece cut out use a belt sander to grind it down to your marker line. I use a stationary 72" belt grinder, but a carpenters belt sander would work just fine. If you don't have a belt sander go borrow one. Flip the sander on it's back so you only have to hold the lens. Doing this part by hand would take forever. Using a sander you'll be done in 5 minutes. When you start to get close to final size start checking the fit. You want it to just slip inside the mag bezel. Be careful to sand 90 degrees to the face and back so everything is square. you don't want any taper. Take your time and you'll get a great fit without gaps. Now that you've got a perfect disk you just need to grind the step. This will require some sort of jig. If you have a router table or a drill press this should be pretty easy. For example, if using a drill press I would clamp something to the table to form a 'V' shaped fence that you could push the disk into. Then you could turn the disk keeping the cutting bit at the same depth all the way around the disk for a perfect step. Be safe and all that. I accept no liability for detached digits or other bodily trauma. 
-Ben
ps - don't peel the plastic off the polycarb until you're done. It scratches easily.
