Maple
I'm curious as to why you used maple for your cores (other than convenience)? I know almost nothing about musical instrument pickups, but usually I see transfomers, solenoid coils, etc made with cores that have strong magnetic properties, such as iron. On those it makes em much stronger than something like an air or wood core.
Short answer - convenience - but I don't think you'd want ferrous metal cores unless you had a good idea what the resulting magnetic field looked like.
From my understanding from the Pickups book, the magnets establish a magnetic field, and the vibrating strings make subtle upsetting of the field, which is then picked up by the coils of wire. The cores are just to hold all that wire. Telecaster style pickups are wound right over the cylindrical magnets but the ends that hold it together are plastic.
I ended up with the maple from trial and error. First I tried the plastic(?) fiber that Stewmac uses in their telecaster style kits, intending to wrap directly around the magnets. After laying it out, drilling and putting in the magnets then winding 4000+ turns, when I went to tape over the windings per the instructions, and wanting to get it nice and tight - pop went off one of the ends and it was a bird's nest. Dang.
So I looked with more favor on the idea of plastic cores in their humbucker kits, but I don't have any plastic so I duplicated them to the size and spacing I needed out of the hardest wood I had around - more maple. So with the magnets inserted into tight holes the core rather than touching the wire, I could press them down to set them just right after they were in the uke & strung up.
Now I like the looks, and also like the look of the red copper better than black tape over it.
The experimenting will continue for sure.
-Vinnie in Juneau