Nice one, i made one with a c core inside a case with electrical connections... i use a bolt to change the inductance... the coil is fixed in a way the its half fixed to one of the cores...
i used pieces of antena to make it slide perfect and springs to make it very fine tunable..
In my case i connect it to a tap in the choke... and the "isolated ground" point...
with my pll i was able to change the frequency of the resonance by changing this value and the pll followed and kept at resonance without changing any other button... i could vary in one test from 65khz to 110khz...
Compliments !