I’ve got several favorites:
1) plain old scungii hair-tie. I double it around my cock, or once around cock and balls after full-package pumping. Cheap, plentiful, totally stealth.
2) 1-1/2” stainless steel cock ring. These have to be *very* carefully sized. Amazingly comfortable though, once you get a good fit.
3) Smallish silicone ring. This one’s about 1” ID and 1/2” wide. Comfortable for long term wear under clothing, is the least pressure and therefore the least effective at maintaining a pump. But you won’t have to run to the bathroom in pain if you’re wearing it out and it gets uncomfortable..
4) 1-1/4” rubber O-ring. This is a sex shop item, and softer feeling than actual hardware o-rings. It’s my go-to and fits perfectly.
5) latex tubing and bead. These are sold in sex shops, but your local hardware store or big-box has 1/4” latex tubing, and you can buy a drilled bead at a craft store. They work okay, and I used one for a long time. Probably best for pumping full package, as you can adjust the sizing to match your particular gains that day.
Skinheadbig, I hope some day you’re comfortable enough with yourself to share your efforts with your girlfriend. It sucks to feel as if you must sneak around.