One could use the type of clothespin with a strong spring(or whatever mechanism) and attach to either (or both) the bottom or top of navel, with one row of teeth 'inside' the navel and the other one on the outside. Kind of like how piercings sit, though having thouse pins left on can hurt quite alot. I guess (hav e no idea) some might not be as sensitive as others but for meself it does hurt in a kind of an unusual way.

There's also the type of aligator-clips which are slightly larger, which means one can have more flesh in between them :P.. which can work well too, though more painful.

Well, that's as far as my knowledge/experience on this particular topic goes, hope I helped.