If you are going to layer a shoot that is near thickness of a cutting why not just take a cutting? My first thought in seeing your air layer was "this branch is really thin for air layering". Not suggesting that it is impossible to do on a thin shoot, but I think I have heard that the thinner the shoot the more difficult the air layering process will be (all other things being equal in terms of genetics) and less wiggle room you have in terms of the technique to create the girdle.
That said, if it was me, I would let this tree grow on root stock for a few years while shaping each of the areas that you intend to air layer. Sure you will lose a few seasons of nebari development, but you will gain more vigorous development of the trunk and structure due to being on rootstock during that time. You can maintain the mother tree on the root stock indefinitely and grow out multiple potential future air layers until you have closer to a 1 inch thick trunk and then proceed with separating the pieces that are most interesting.
But I also love and appreciate seeing every possible strategy of handling maples and the fact you've already applied the girdle means you are committed at least for this particular shoot. So I would definitely follow along with your progress. I do like the idea of Sphagnum Moss. Scott Elser did a presentation on Air Layering for the GSBF YouTube channel and I saved this diagram of his recommended container preparation. Sphagnum moss in contact with the girdled trunk seems to be a best practice at least in my learnings:
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I don't know what the magic sauce is for sphagnum either, but I have two "Seigen" air layers that have just started to show some roots and I use the plastic wrap + sphagnum method on those. I say "Seigen" in quotation marks because in reading the other thread about genetics for Seigen it sounds like Round Valley material may not be true genetic seigens so I don't know exactly what to make of that. The tree was sold to me as a Seigen and it is producing roots now:
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The girdle was applied on May 23 so I would say this is a bit slower than I would have liked, but perfectly reasonable time to root. I use Clonex for rooting hormone.