Even further progress

 The two-layer neck guitar is progressing further. Now, it's a very nice, compact and well-sounding instrument. There are still improvements to be made, but now that it's playable, I hope to play more and modify less.

The neck is very flat (as Rick Toone's Exoskeleton neck) without any wood on the back. I might add a piece of wood to make the neck thicker and more familiar, but for the time being, playing the thin neck is fun and works surprisingly well. My thumb still creeps up over the edge of the neck, and that's not the proper way to play it. As long as you play with the "correct" classical grip with your thumb behind the fingers, having a thin neck feels fast and precise.

I've mounted two of my "current transformer" or CT pickups on the guitar. There's not enough room underneath the strings for a traditional pickup. The CT pickups work in the same way as Lace's Alumitone pickup: The part beneath the strings is the primary coil, which is connected to the secondary coil (the current transformer), which is at the side of the main aluminium beam. Magnets are small cylindrical neodymium magnets mounted in hex blind screws. Since there's only one or few primary windings under the strings, such pickups can be made very flat. This in turn allows me to keep the whole instrument rather thin, since I don't need deep pickup cavities.

On my neck pickup, I use a brass coil with only one primary winding (made from a 20 x 3 mm brass bar) connected to a secondary coil by an iron core (the green florist's wire). The secondary coil is home made using a sewing machine bobbin. The bridge pickup has about 15 primary windings running through an AS-104 current transformer. They supplement each other well, the bridge pickup being quite snappy and sharp, while the neck pickup is more deep and soft.
 
Both pickups have low impedance; in the range of microphones, which is why I am using an 500 ohm -> 50.000 ohm line transformer before plugging the guitar into my amp.

Also added is a string retainer at the headstock end, keeping the strings from rattling. A bracket for jack, volume pot and two mini jacks for the pickups makes it easy to plug it into an amp and to switch the pickups. For intonation, I've added a couple of screws to the saddles going into the end piece that holds the tuners. By turning the screws, the saddles are moved back and forth.

I still haven't added piezo pickups, partly because I haven't figured out where to put the pre-amp and controls, partly because the CT pickups function so well.

Next step might be a thicker (6 mm rather than 4 mm) plastic body, this time bent with a little more care. The one I'm using now has been bent into shape using a heat gun and brute force. Another improvement could be a piece of wood on the plastic body that acts as forearm support - the curve of the body makes it hug the torso of the player (which is nice and gives good control of the guitar), but there's nothing on which to rest my arm, and that becomes a bit tiresome.

There's only one aspect that I'm not quite satisfied with: The bow of the neck is not easily adjustable. This is due to the strength of the beams - even without tightening one against the other, the pair of them are enough to counteract the pull of the strings, so there really isn't much to adjust. A solution could be a thinner back beam, but I am not sure they are available in the necessary width.