Lots of good comments here.
My favorite is stainless steel, I wear a 40 mm 1.57" for flacid daily use with a split ring and a padlock that came with my bon4 cage. It is nice that the bon4 comes with i believe 5 different size rings rocked out a 2" works nicely.
I have tried the silicone lug nuts and really liked them, however over time they get stretched out and become loose and eventually just break.
I also have bon4 magnetic ball stretchers which as commented earlier can some time pinch when putting them on, they sometimes come apart during different activities or working out.
I have a variety of split rings with allen wrench closure design which I like for ball only wear which really gets them swinging, the largest which is over 2# is great for some ball breaking fucking lol.
Last, never underestimate your hardware store, many metal rings and shackles as well as O rings discussed earlier not to mention leather boot laces which have an infinite ability to be sized and wrapped for comfort.