Cut a piece of clear plastic the size of the backing. Cut four pieces of plastic to create a frame around the cells. These pieces should be thick enough that when they are arranged around the cells that once the plastic cover is placed on top the cover doesn't touch any of the cells. Make sure that there is a small opening in the lower right side of the frame to allow space for the two connector wires to exit the panel.
Attach the frame pieces to the backing with a small amount of adhesive along the entire length of each piece. Position the connector wires to exit the frame. Once the four pieces and the wires are secure, place the plastic cover over the panel and drill holes for the wood screws. Make the holes in the plastic just a little bigger than the screws to keep the screws from stressing the plastic. Screw the panel in place and seal up any joints, especially where the connector wires stick out. Adhesive or silicon around the sides should keep water out fairly well.