Some mechanics, some basic soldering and some playing with capacitors.
I started at 680pF parallel to the tuning capacitor, resonance was a bit too high. The additional capacitor changed to 820pF resulted in a perfect resonance (503kHz) in the mid-range of the polyvaricon :-) YES!
Here is what the tuning/coupling-box looks like:
Funny coincidence, both capacitors I tried, came from the same dead CFL.Oh, and BTW, I used a 6:1 toroid transformer this time...