Actually you can rig any fishfinder as a portable. I have a Hummingbird 727 that I used that way. It was mounted to a piece of 1 X 6 which fit inside one of the waterproof plastic ammo boxes and the 'ducer was fitted to a piece of plastic PVC pipe I hung over the front of the boat when I was spider rigging. You could see the screen just fine in the ammo box and when I was longlining I didnt carry that fishfinder with me, just used the one on the console.

I have the HB 727 for sale for $200 plus shipping but between me and my son we've both used the Eagle 320 and 480 df's and I can recommend both of those as well.