Inside the Little Plastic Castle

I don't know how useful my input will be: I make lace with fine-gauge yarn, and anything that can snag the yarn or tangle in the openwork, will. Consequently, I prefer absolutely smooth, closed, undecorated rings. My preferred size varies with needle diameter, but large enough that I can comfortably put a needle-tip into the ring while it's still on the body of the other needle, so... inner diameter maybe 2x the needle diameter?