For myself, I just cut to the chase and bought the Hobie Fish finder III installation kit, P/N 72020070....it's complete with a 9 amp hour rechargeable gel cell battery and charger, the wiring is long enough to reach forward....I dropped the battery in the aft 8" compartment with the custom fitting battery holder and added an inline fuse....fished the wires forward with my Stanley 1" wide tape measure.
It's quick, neat and the battery/everything is of high quality.....done deal....My Humminbird 768 fish finder/GPS combo will operate for more than 22 hours w/o recharging the battery.
You wouldn't happen to know the amp draw rating for your 768. The Elite5x is claimed at 1.1a but I'm being led to believe it'll be less, especially without much backlight.
I keep looking at the Hobie setup you got...likely may just do the same. The 5x will be here, finally, Monday. The only thing that's held me back is I could get the same/similar battery for a whole lot less. That way I'd have to rig a 8" tray liner to house it though, but I wouldn't be so much locked into a battery size.