If you're not going to be fitting in a round lens, you can simply cut any hole and glue (GOOP) any shape glass over it. You can put the glass inside or outside, just using enough glue to smooth the sharp edge. If you use a camo finish, you'll be covering the outside edges anyway.
Round galss may be hard to find. I checked several welding supply stores for these. They have 50 mm plastic for cutting goggles but not glass. They said they no longer sell glass since it's considered a safety hazard on construction sites. I had a bunch of scrap glass and cut a lot of it up into arc welding lens cover plates which are roughly 2X4. I just cut one of those in half and goop it on the inside.
I used glass microscope slides (unused, clean ones!) for my lens, flash, and viewfinder/flash sensor openings. They are optically clear, probably have less distortion than window pane glass, and are easy to cut with a glass cutter (...only had to trim the length a bit).
If you know a friend that works in a lab, medical office (some still do some of their own lab work), or a veterinary office.....then I would ask them. Make sure that they ask for them instead of just taking (stealing?) them. PLEASE MAKE SURE THAT THEY ARE NEW, CLEAN, UNUSED SLIDES....DO NOT ACCEPT (FREE) USED ONES! YOU HAVE NO IDEA WHAT WAS ON THEM! Also, hobby stores, like Hobby Town, that sell science stuff for kids should have them. They usually come in boxes of 50-100, and are very inexpensive.