Build your own. I took an older Bottom Line DF and mounted it on a plastic box. Put a 12V battery in the box. Mounted the transducer in a block of foam (picture is the first block - the present one is much smaller). I am doing the same thing to my new Helix 5 DI GPS with a larger battery.
The battery in the box is a 12V Dewalt Drill battery. I had the drill and two batteries and pressed it into service since I never use them both at once.
I drag it behind my canoe and my jon boat with a TM. Works great.