Built a rollback can and wanted a transparent cap so students could see what was going on inside the can. This is for a 16oz coffee can. I also have a laser cutter, so my design used a laser cut .12 thick plexiglass "window" that nests into 3D printed rim. Also designed (but did not test) a 3D printed cap for those w/o a laser cutter. Rubber bands held in place with standard unformed staples. I used 2 rubber band to ensure mass was centered in can.