I wanted something easy and portable. I bought one of those ammo boxes and put the battery inside and attached a Ram mount on top to be able to attach and de-attach the unit to. Then drilled a couple holes plugged with rubber grommets, one for transducer cord out the side, and one for the unit on top.
For mounting the transducer: I use one of those YakAttak tube style rod holders which slides onto the track mounted on the top left side of my kayak. Then I used one of those flexible bendy bungee type of things where you can bend them around something. I run the transducer through the rod holder and attach at desired angle and depth with flexible bungee thing.
I love it because the whole set up can be easily stored, used in my other kayaks, or elsewhere, and requires no drilling or gluing in my kayaks. Best of all I like having the unit itself on a movable ram holder right at my feet to where I can easily adjust angle, view screen or touch any controls.
My description may sound complicated but it was actually very simple to make and easy to use.