I have only ever used clear plastic. _It helps to be able to know when to judge watering and remove the layer after the roots lignify and turn a bit brown.
Most of what I have read suggests waiting until the spring growth hardens before setting up the layer, but since you're using a garden tree, I think it might be worth trying an experiment and testing whether you get better success with early or late season layering.
Books are fine, but until you get a process dialed-in that works for you, it's hard to guarantee success.
there are some nice diagrams here:
photos & how to:
http://www.bonsaitree.com/bonsaiaid...les/abal_1.html
diagrams:
http://aggie-horticulture.tamu.edu/...r/airlayer.html
but the advice with regard to making a slit in the branch rather than ring girdling... take into consideration that the second article is talking about a basic plant propagation process, not getting a perfect radial root system, so I would go with the ring girdle. _And wait a while longer on separating the layer so you can be sure it will survive.
More info in
bonsaiTALK Links under techniques-> propagation, and also the ***FAQ***
Regards,
Matt