I found a lot of these online but they didn't quite resemble the real thing enough in my opinion. I tried making my own as true to the original as possible.

It has an inner diameter of 50,8mm with 1mm bumps along the inside diameter for a snap fit. Still a work in progress for the perfect fit so I might update the files

Don't mind my crappy paintjob, the pictures are from the first try (too tight of a fit at 50,2mm) in PLA. Planning to print the final model in TPU but haven't got any grey at hand…

Use the step file if you want to get the smoothest surface, the STL is a little bit choppy.

EDIT

Seems like the 50,2mm actually was the way to go for a tight friction fit. I removed the stops so it can sit at any height tightly fitted to the tow hitch ball

The loose version is 50,5mm and that has 1mm stops which squish a little at first application and make for a quite loose fit with snap in place action