I needed a box that could be installed and taken off in pieces. Here you can see the top, front section, and two sides. The back will be open for ventilation. All were made out of 1/2" plywood and primered inside and out with two heavy coats and then I added two heavy coats of gloss black enamel on the outside.
This is a picture of the batteries mounted in between the frame rails and behind my propane bottles.
The frame that I built holds the batteries apart with enough clearance for air circulation for cooling and the small unpainted square of plywood holds them down via an all thread rod that extends up far enough above the batteries that it will also hold down the top of the box.
This photo shows the side that are notched to hold them in place via the battery frame and the front that is notched so the trailer frame rails is hold it in place.
The sides extend to the bottom of the batteries but the front has an air gap at the bottom.
This picture shows the top mounted ( it has 3/4"x 2" strips mounted on the bottom side so that the front and sides are sandwiched between them and the batteries) and now it is ready to remount my propane tank cover.
This provides protection from the weather but allow air circulation and easy access to check the water in the batteries. The black makes it blend in with the frame.