I needed a cap for a square metal pipe to keep things out of it and protect me from it's sharp edges.  The square pipe has an outside (OD) of 40mm and the cap is just a press-fit.

Here is my 3D design.  My design may match your needs, too.

My print is made in PETG 1.75mm Aurarum filament, with 0.2mm layer height and 15% infill on a PRUSA I3 MK3S printer.  No brim nor supports required.

I hope you can find this design useful, too.

Remember to leave me some feedback or even a tip to keep me enthused to make more of these drawings.