I don't use a fish finder but if I did I would seriously look into the Ram transducer arm set up that deploys your transducer over the side of the kayak, along with possibly the YakAttack Cellblock. Pretty slick set up that can be easily taken off and left at home or locked in the truck while you go in and grab a cheeseburger. There are lots of other ways to set it up or make your own transducer arm, just do some searching on YouTube and you'll find lots of options in all price ranges.

https://www.youtube.com/watch?v=kajYeTUmbdc