1/2" plastic plugs by Caplugs this plug is a flush slightly raised head that measures approximately 3/4" while filling a 1/2" hole. These plugs will fit snugly into sheet metal, but will need cemented in thicker materials either with 3 M body sealant or silicone or an adhesive of your choosing. The inner ring that snaps through the sheet metal is slightly larger than 1/2" by a few thousandths of an inch to make them snap in so to speak so they may be difficult to push in I would recommend that you use a little bit of liquid soap if you are having an issue. These plugs are made of LDPE plastic. And give a professional looking finish. For use on flat surfaces.

BPF Series

Button Plugs with Flush-Type Heads