After making my own for many years, I finally gave up and bought the air layering pods from Amazon. They make the air layering so much easier. I bought these for about $1.50 each. They are totally worth it.
I like the ones I bought. They are made easy to even water the pods if you wish. There are also inspection ports on both sides so you can see whether the roots are coming. I put one on my rainbow eucalyptus about a month ago and I already see the roots showing at the ports.