Here is a useful model for printing button hole fillers for metal arcade control panels. Just pop them in from the back, and cover the back with duck tape or something to keep them from falling out from the back (they will not go forward as they are designed larger in the back so they won't go through.).
This design keeps them flush on the front so control panel overlay can be applied without having visible "bumps". you could put them in with the larger side on top instead of the bottom and then no tape would be needed, however then they won't be flush for when control panel overlay is applied.