I am using a Humminbird 788Ci Combo GPS FF (had a 385 combo but I upgraded to the larger screen 788ci). works well and the screen is big enough to see easily. I have this FF mounted in Mad Frog's "liberator" mount that has a transducer arm mounted to it, the Liberater is mounted to a Scotty FF holder which in turn is set into a Scotty rail mount on my Pro Angler side rail. no holes to drill through the hull and everything except the battery is all on the Liberater mount. I use a small 5 amp 12 volt gell cell battery in a waterproof box.
http://madfroggear.com/liberator/liberator.htmlthe only disadvantages to the setup above was the initial cost of the FF (expensive!) and I had weeds get stuck on the transducer arm but I can easily reach over and flip the arm over to remove the weeds.