Have got a Garmin FF 90 on my swing. You can see the head unit position below. I remove it and stow it in the front hatch in a bag when going thru surf, only takes a sec and prolongs the life. Even if they say ipx7 alot of fish finders dont handle waves crashing over them. My 12v 7ah battery sits on the hull inside a foam cutout with 2 securing velcroe straps. This is accessible from the front hatch. My transducer sits on the hull under the small hatch between your legs. I have selleys allcleared an open pipe end which the transducer sits in and I top it up with water each time I go out so its always looking thru the medium of water and there is no chance of air bubbles in goop etc.

Here's my Eagle Cuda 300 I have just mounted on my SIK.

Greenman most people use the 12v 7ah and this will give you plenty of juice to fish all day.

Greenman where are you, if in Auckland send a PM to infini on here, he'll sort you with a battery way cheaper than that...