That’s pretty much the idea, except it could be transparent so you can watch. Make it in various shapes. The cup sizes are determined by research. My best guess: time is inversely proportional to sucker size. A smaller suction cup will be comfortable longer. Each person will account for his/her pain threshold, and choose the size accordingly.

It's a timer which uses yourself as the alarm. Stick a Time Sucker on your body, then have a quick nap, start water boiling, etc. You will definitely know when time is up. And you'll have a reminder spot.

Note this isn't for pulling innards from one's body [snarfyguy's link] -- gross -- Are those bell jars?? It's more about a craft store soft rubber slight-vacuum version like at po's [link] mail-order suction cup shoppe.