This parametric model generates a plug or cap with external threads.  Customize it here on Makerworld, or download the Fusion file and do it there. You'll need to know the Major Diameter, Minor Diameter, and Pitch to create the right threads for your plug. Measure these with calipers and be as accurate as you can be. I also have a Clearance parameter you can change to make the fit looser or tighter. Photo below shows what measurements you need (but doesn't show a depiction of the plug).