A set of four knobs to fit these twist pins available on Amazon https://smile.amazon.com/dp/B07QLL9W8Z/ref=cm_sw_em_r_mt_dp_U_m7seEbX5GT2YT
Use a drop of CA glue and then press the head of the pin into the knob.
These pins work great as a substitute for staples for securely holding paper targets to backers made from polystyrene foam insulation board. We cut these sheets into 2'x4' sections. Each section has plenty of room for two shooters. https://www.lowes.com/pd/Kingspan-Insulation-Common-1-in-x-4-ft-x-8-ft-Actual-1-in-x-4-ft-x-8-Feet-R-5-Unfaced-Polystyrene-Foam-Board-Insulation/999972966
TIP: Revere's Riders recommends that you reinforce the edges of the foam board with 3" wide Gorilla tape. We cut these sheets to 2'x4' and secure each section to a couple of plastic step in fence posts using zip ties. These will handle 10's of thousands of rounds. Happy shooting!
Check out Revere's Riders for top-notch, affordable rifle marksmanship training: http://www.reveresriders.org