I just shoot through the hul, put the battery in a dry bag in the front hatchl and pull the transducer and battery cables through the front hatch (and then close it). I then have the head unit mounted on a piece of wood and bungeed to the front hatch. I attach the bungee hooks to the hinges bungess and the loop is attached to the normal front hatch bungee closure element (I do not use the closure bungee attached to the front hull). No drilling, no fuss, great performance, no commitment. I think it is the best solution, but to each their own. You can probably see it in most of my fishing posts.
Fish tremble when they hear my name
A ship in harbor is safe -- but that is not what ships are built for.
--John A. Shedd, Salt from My Attic, 1928