Not being familiar with ferro-cement boats, can I assume that it has some metal reinforcement that runs through out the hull
? Is this metal all tied together? Is it isolated from other electrical
systems within the boat? Can you access it to wire to it?
If the answers to all of the above were "yes" then I would simply connect to the metal structure of the hull
at the nearest point possible to the tuner.
If the answers are "no" then the easiest solution, if you have one handy, would be to run a copper strap to the nearest bronze through-hull.