A couple of months ago we went to a sexpo, where we picked up some nipple jewellry for oh, there was loads on this stall, some just for nipples, some with chains to join them together, and some that joined on to a necklace, anyhow the bits that fitted on to the nipple are like a noose kind of thing, so is fully adjustable, and the ones we got the wife can wear for hours, sadly it would be against the rules for me to name the company, so you would have to do your own research to find the stalls web site.
I've had a look on here, and the only thing I can find that is similar is the, peekaboo ruby and diamond star nipple loops, (sorry don't know how to do the linky thing) if you look on the bondage/nipple section you should find them.
These look like they have a spring system, rather than the noose system on the ones we have, so don't know how comfortable they would be for long term use.
I would only suggest clamps for short term wear, as in my experience the longer you wear them, the more they hurt when removed.