Find the top to a pen that is circular and fits the camera lense. I have one or two on my desk that come pretty close. Hold that down on your screen protector and cut around it with a razor knife.
Go to office depot/staples/etc and get one of the plastic architects stencils. They normally have a few different sized circles you could use. Then use a razor knife to cut it out.
Finally, you could wander around Home Depot/Lowes and see if they have anything. I would check the tools (maybe drill bit area??) or hardware or that random isle with signs, mailboxes, etc.