Sometimes the manufacturer of a boat provides the bare minimum of what the boat needs to be operable. This customer wanted to be able to live more comfortably while away at anchor. The issue with the stock boat was the the twin engines had a battery for starting each engine. The house loads were drawn off of one of the batteries. That battery quickly emptied while at anchor.
The fix to this issue was to add an additional large battery (8d) as a dedicated house bank.
There was only one place to put this battery, The picture below shows the installed mounting brackets. Since the boat was launched early, Glassing in a set of brackets was impractable.
This space was just big enough for the battery box to be installed and be under the rudder arm (the stainless bar on right) clear of the access to bilge pump and above the bilge water, The fiberglass angle brackets will not decay and are rated for much more than the weight of the battery and dynamic motion of the boat underway. Luckily the bolts attaching the brackets were above the hull and I was able to get nuts on the bolts.
In the picture below the battery box is placed into position. It wasn't easy fastening the battery box to the brackets. With long screwdrivers and rachet drives I got it done. The fastening of the box is done on the outside corners (hard to see in the photos). We never screw through the bottom of a battery box. A battery box is intended to be able to contain battery fluids should the battery leak. Any holes in the box violate this requirement.
As you can see in the above picture the mounting bolts are just above the hull connection to the carlin.
See the gallery below for the rest of the project. Click on a picture to enlarge it.