ELONG nip-suckers work, they really do suck...and look so cool. They are slightly curved but not tapered--these will suck any size nips and are deep enough and big enough for small to medium piercings.
These suckers stay on, the base flares out for more contact with skin, they will seal to small or larger--even furry chests.
What give these powerful suction are the four springy rings that make the nipple-tube spring open without collapsing like old snakebite suckers-- we made the walls thinner to allow for more pump and even larger sucked nips.