A little slow in replying sorry,
The battery bag is inside the hatch. I glued some velcro strips on the side of the yak inside the hatch. The battery bag which is the drybag I carry the entire system in (I use rechargeable "AA"s for now) is stuck to the velcro strips under the cup holder/mesh pocket. The cables come up through a drain plug with rubber stopper in the mesh pocket. I have my fish finder installed as removable so if not fishing it is not in the kayak. The tranducer sits in a foam block that was glued to the hull with Marine Goop and I add water. The setup works like a charm.
here are some pics
the foam block under center hatch: tightly holds transducer and just add water. I took off the mesh for clarity to see the RAM mount and the drain plug - note cap is attached by string tied to RAM base so won't lose it and the rubber stopper is attached to that string ready for use by a small brass eye screw.
The drain plug - cut off the cap keeper and just use the cap. Drill a hole for the string.
Next two pics show the portable system in drybag and the components:
The battery cable & transducer cable is fed from inside the kayak up through the drain plug and then the rubber stopper which is pushed into the drain plug opening leaving only enough cable out to reach the fish finder.
The fishing setup and note the anchor is wound on a cuban YOYO, no more tangles, just unroll anchor line cleat off then clip into anchor trolley and you are set. To retrieve just wind back onto the spool wrap with a mini bungee (to hold anchor to yoyo) and you are done.
When not fishing I remove the entire setup and screw the drain plug cap back in. This setup has the fish finder close and easy to read/operate buttons (in lieu of being in the mast hole at my feet) and when not in use the small RAM ball & drain plug take up practically no room in the pocket to be of any consequence.
The RAM mounts I bought at Bass Pro Shops or online at Kayakfishingstuff.com.
I hope I covered all the bases.