Continuing the story, I needed a neck cavity I made hell-of-a-mess with that, as the upper side was a really hard wood, which resulted in the wood burning and smoking all the time. But it turned out like this, and became even better after some chiseling.
From the next step I have learnt that I should think first and cut afterwards. I was thinking that I don't want humbucker ring to obstruct the top, so I wanted a "back-adjustable" humbucker. Not sure how I thought that, but (fortunately before cutting the rear access) I realized that humbuckers are meant to be front-adjustable (screws, spring, gravity) .... but the center seam was already cut, so no place to screw the neck HB in, so change of plans to a soapbar P90, which has the screws in the middle, which would be on the center-seam. So "quickly" ordered a set of soapbar P90s along with some more accessories, including a telecaster Jack. Fast forward 40 days, when I get to pick up the ordered parts... and it's a 25g package, which only has the telecaster jack (mid-March). Nice one. And in the meantime coronavirus hit, so no chance to order another set and get it soonish...
So checking what I had in-house: two humbuckers, 3 single coils, 3-piezo disc acoustic pickup and a soundhole pickup, which didn't work as a soundhole pickup due to a broken side-ear.
Humbuckers and single-coils had the same adjustment screws... so I was left with the piezo pickup and the soundhole pickup (something like a Seymour Duncan Woody, only made by Shadow). The soundhole pickup had a 6mm thick wooden top, which I cut down, to "replace" it with my plywood top. Also attached the piezo discs to the top, attached the magnetic soundhole pickup to the top with two screws, and made some tests to check the volume of the magnetic and find the best location for the piezo discs. At this point I only had clamps holding the two layers plus the top together, but the body seemed to be resonating well.
As the top is not really moving freely as in acoustic guitars, I found that I'm better off putting the piezo discs inside the body (not on the top, but inside the chambers). And one of them sandwhiched between the top and the center seam close to the bridge.
Oh, and I almost forgot: wanted through-body stringing (to reduce the parts obstructing the top drawing to the bare minimum), but as the body is fairly small, full-through-body stringing would've required very steep angle on the strings, so I have drilled from the side to the top, drilled a bit larger on the side to fit the steel nuts I was using instead of string ferrules (no string ferrules are to be found in Romania, and Corona was delaying every import).