If you do not need mapping I would look at the Garmin Striker series. They have a lot of great features at a affordable pricing but they only have GPS.
I have fished out of boats like yours my entire life until I bought a 17 foot boat last year and I always had my depth finders removable. I would mount them on a piece of plywood and use battery clips on the power cord and used a suction cup style mount for the transducer up until I found the Sully bracket. I never had any problems and still have a Lowrance that I will someday put a small flat bottom boat for my son to use.