Hey, when is the best time to air layer maples like the Sugar Maple?

Last weekend I air layered a branch on a Sugar Maple that is in my Aun'ts yard. I thought maybe when the cloraphyl in the leaves start to move down to the roots as they change colors sinces it approaching fall will be trapped in the layer and develop new roots.

I did the gurdle method where you cut off a ring of bark from the branch and you coat it with some rooting hormone. Well I haven't wrapped it yet and just left the wound open to air with rooting hormone on it. I'm going to cover it with a plastic pop bottle with regular miracle grow potting soil in it.

Is fall a good time to air layer a Sugar Maple or should I have waited until spring? How do I keep this air layer from freezing on the tree this winter? I live in northern Missouri which is zone 5 and it sometimes gets pretty cold here. Please let me know. Thanks.