About the worst thing you can do to a high Q loading coil is to put it in a metal box. its like a single short circuit turn on the coil. It will kill the Q factor and probably get very hot.
Also in their case, using multistrand litz wire will help as it reduces eddy currents in the coil itself
Some of the NDB aviation beacons are no taller than 10M and operate down at 300 khz so it is possible to radiate something on those frequencies with very short antennas. I dont know how much power they push into the aerial or what the ground plane is like but they are audible over 10's of miles so there is some hope that arctic net could achieve something although I doubt we will hear it here.