You can get packages of 3/8" aromatic cedar tongue and groove closet lining at your local home improvement store. Since most boat hanging lockers are tiny by household standards, you won't need much. Then it's just a case of cut and fit. Well, finicky cut and fit.
You can install it with stainless finish nails if the substrate is wood, or adhesive if it is fiberglass.
- Before you start, be sure that the door is wide enough so that you can wedge your body in there far enough to reach all of the back wall. Or find a small child to do the work for you.
- Purchase the cedar lining and store it on the boat , unwrapped, for a week or so to equalize the moisture content of the wood with the boat environment.
- Do the back wall first, working from top to bottom. Your first board should be square with the two walls. But as you go down, it is likely that things will get interesting, since the wall is probably the inside of the hull or hull liner, and therefore is curved, and is wider on one side than the other. Make the boards 1/8" - 1/4" shorter than the space they go in, to allow room for growth due to humidity. The gap will be covered by the thickness of the boards on the sides.
- Next do the sides, making sure that the joints at the back wall are tight. At the ends behind the door, where they will be least likely to be noticed, again provide a gap for humidity growth.
- If you are using adhesive, you will only be able to do a few boards a day. Tape and wedges are your friends.