Hello Art,
I have not air layered a fig trunk, but I have rooted one as a cutting, about 1 inch in diameter and 24" long. Fig plants are very resilient and as long as there are roots and leaf nodes (growth points) they will grow. Attached is a edited copy of a picture that is posted on this forum, of a "Japanese fig espalier" for growing in containers. the cross hatch on the branches and trunk represent leaf nodes. I edited out the metric dimensions. Step 1 and 2 represented one growing season June to September of this year. The spread of the main branches can be adjusted as required. If you do a search, the available info is quite descriptive. The ultimate shape of the tree is up to you. Also note the attached picture of the step-over form for in ground planting (possibly next to a fence).
PS. This would not work for breba only cultivars (Desert King).
Good Luck