Hi Chris. Thanks for update, and in general all your recent informative posts. My guess is that the new air-coil is probably 18ga, similar to this:
Jantzen Audio 4.0mH 18 AWG Air Core Inductor Crossover Coil
15ga version will be massive and heavy, probably overkill, although it will have much less DCR.
Will be useful to confirm. Not that I will modify the backplate, but good to know.