I doubt the making of 4-layer PCBs will ever really be DIY, but sure is a nice project. Like others, I'd be interested in hearing what the internal resistance of a coil and phase are.
Also, for outside use, I think I would coat in an extra layer of epoxy; don't think I would trust the green coating to be sufficient protection against the weater.
Where and how did you have the PCB made, and was it expensive? Do they make single PCBs, or did you need to order an entire series?
36 windings is already a lot, but, why stop at a 4 layer. Put 2 prints on top of eachother. Or 3. Or...
Might be a good way to make use of small and weak magnets, since airgap can be pretty small with these 'self supporting' coils.