Thanks for the input everyone! I can't believe how many people have suggested to me that I use tape. Now I can't wait to get my hands on some of this stuff to give it a try! My only worry is how long the "tape" holds up after time.
I usually do single-lined clips for my own daughter's fine hair, because I feel like the fully-lined clips don't slide into her hair as easy. It gets kind of tangled. That said, a lot of my friends who buy these from me are pretty "posh" mommies, and they prefer the double-lined clips because they think they look nicer.
So, I make both. Single for my daughter, fully lined for my friends.
Thanks again for the help. If anyone else has suggestions on how to do the fully lined with a glue gun, I'm all ears.