I bought a couple plastic ones (3.99 each) with my Riva starter kit and they work fine.. as others have said, I have to remove these before I drip (2 or 3 drops) in there then snap it back on.. Not a big deal for me (except the o-ring might get cut or ripped by removing it too much), so you might want to spend the money on the metal ones if that is what's right for you.
