Pick a convenient corner, since using a corner means only two walls will be required, and use your measuring tape to measure the length and width of the storage area on the deck. These two measurements will be the horizontal dimensions of the walls. Measure the height from the deck to the overhead to determine the height of the walls.
Decide which of the two walls you are adding will have an opening for a door. Depending on the type of storage, you can made a wooden door or hang a track for a folding or fabric door, or even a canvas curtain. Install a battery-powered boat interior LED light on the overhead near the center of the soon-to-be-built room
Cut plywood sheeting to the height and width required with your circular saw. Cut the opening for the doorway in the piece of plywood that forms the wall where you want the doorway. If possible, cut the doorway as a single piece of wood; this will allow you to use it as the door if you so desire.
Sand and paint, or otherwise finish, the plywood. Allow the finish to dry, if necessary. Move the walls on board the boat and secure the plywood to the deck, overhead and walls with 90 degree angle braces, screwed into place. Use 90 degree angle braces to secure the new walls to each other. Use wooden or plastic corner caps, cut to the correct height over the joint between the walls.
Install the door of your choice to cover the doorway to the new compartment. If the door is a hinged door, remember to install a latch or knob so the door won't swing open while your boat is under way.
