Photos showing how to install a fish finder: 

Profish 400 

Parts to fit the transducer 

Will assemble in this order once installed

Remove bung, seal screw holes and drill hole for cable

You may need to trim the transducer like this Garmin one

So it looks like this and fits the recess

Fit the transducer to the mounting bracket and put the small disc on

Pull the cables up through the hole and place the transducer in position

push the foam plug down the hole

Fill the hole up with "MS All Crystal Clear" (Holdfast brand)

Place the disc with the cable slot on top

Now put the last disc on with the washer and nut, don't over tighten the nut - tighten so the transducer is held in place and doesn't wobble, it doesn't need to be tightened further.

thread the cable through the hole and fill up with sealant and screw the cable cover on.

drill a hole and bring the cable up then put sealant over the hole  

Put the cable cover over the hole and clean off excess sealant

Tidy up the excess cable

The battery can go inside the hatch by using an extra deep hatch bucket that can take a 7ah SLA battery (or Lithium FPV battery).