On my last boat I made a fish finder mount hat was held in place with a C clamp. It worked quite well on my boat as long as I was just using the electric motor. I tried it on a friends boat that had more freeboard, but it couldn't reach the water. Someone with more skill with wood than I have could probably make something better, but my main issue with it was that if I used it with the outboard it would direct a ton of water into the boat. Again, someone with better skills that I have might be able to get around that issue as well.
You can go through all of that trouble or just get one of these.
http://www.cabelas.com/product/Hummi...w&gclsrc=aw.ds
