Eddie was pointing to a source of free ferrite toroids, hence, I took the chance and used one for the Octaplumb. The toroid came as 10:3:3-transformer. To simplify matters, I removed the two low turn count wires and kept the 10 turns winding. So much to coupling....
Resonance is obtained by the typical (see earlier entries) polyvaricon and a parallel capacitance. In this case, 680pF from another slaughtered CFL did the deal getting the loop resonating in the 600m band.
This is what it all looks alike: