I manage to swap a fishfinder for a job I did many moons ago. This is the site I did http://www.tackle2go.co.za/ glad to see it is still up and running.
I decided to make my fishfinder portable so that if need be I could swop it between boats. The first mission was to get the battery into some water proof container, manage to find a nice size clip lock container. I added some foam on the sides and the top to make sure the battery did not move around. I managed to find a water proof plug system so that I could disconnect the battery from the fishfinder with out having to open the whole box. The next time I am going to silicon the wires through the container and leave the connector loose. The reason for this is that when I attach the connector to the battery pack it sits to high and could easily get broken off.
I mounted the fish-finder screen onto a piece of wood and at the bottom of the wood I stuck some geko feet. This would prevent the screen from slipping around the deck. It works like a champ. I got the geko feet from a material shop and I have found 1001 uses for it, it is the best thing since sliced bread.
The next thing was the transducer, I mounted this in some foam that came from a computer box. I shaped it a bit like a boat, cut out the center to house the transducer. I secured the transducer with some cable ties. Added some rope to the front of the foam and a clip on the front of the rope so that I could attach it to my kayak easily. I cable tied the transducer wire along the rope, so that it would pull the rope and not the wire.
Have a look at all the pictures to get a better idea. This setup works so well for me and I have used it on many boats.