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.
Good luck.