This sounds fun! Fun for me I mean.
If you have clothespins, great. If you don't, I happen to know a few mainstream stores have these delightful teensy clothespins available- Michael's, Target. Those look cute and hurt like a bitch.
Alternatively, go to a dollar store or any department type store and pick up a pack of cheap girly hair clips, they can be the "micro claw" style or snap style (to use snap style, pinch the skin along the side of the clip, not in the point), or really any little clips that look uncomfortable.
Use your clips/pins of choice along the underside of your cock, lined up in a nice row base to tip.
Take the number of clips you have attached and subtract that number from 20. That's how many minutes you'll keep them on.
Separate task, attach one clip anywhere on your cock before a brief outing (like running an errand). You won't be able to remove it until you're back in the car or in a bathroom. Any kind of clip will work, but the tiny clothespins are especially great for this task since they have a tendency to twist sideways, suddenly pinching the tiny bit of skin they're holding much more painfully.