I've used hair bands or whatever they are called. You can find them in the female cosmetic section usually. Essentially they are rubber bands with a sort of cloth weaving around them which also expands and contracts. This makes them much wider than a rubber band and also soft on the skin. The large sizes are for holding a ponytail in place, but there are many smaller sizes as well.
They come in all sizes and colors and it may take buying several sizes to see which fits best.
I've found they can be worn for days on end without discomfort, but you do need to change them every few days because of urine spillage. It can build up depending on your habits. One ring is often not enough to keep the foreskin retracted. Two hair bands work better when worn just under the corona.
Aside from the hair bands, you can also purchase metal frenulum rings, but these do not expand or contract with the condition of the penis. Buy too small and they hurt with an erection, even partial. Buy too large, even with a small margin, and they can/will fall off and slide down the leg of your trousers at the most inopportune moment. Or fall off if you're wearing nothing. That said, I have seen these frenulum rings worn at nudist beaches, a place I would not expect.
If you ever order any such ring, don't confuse it with a penis ring, which is worn at the base and over both penis and upper scrotal sac and therefore larger in diameter. These don't come loose very easily compared to a frenulum ring, which is worn just under the corona.