Here’s what we used:
Two 1×8 ft boards
Measure the dimensions of your space using a tape measure. Cut boards to fit dimensions (essentially you are creating a rectangle – ours was made using two long pieces measuring 36 3/4” and two 10″ pieces). The next step will likely be a little different than ours because we took advantage of the molding along the bottom of the wall to use as a place to rest the bottom board. We then nailed the side pieces into the wall, and nailed the remaining long piece on top. If you are constructing this without the help of any molding/wall features, you could use wood glue and clamps to secure the pieces together in a rectangle shape, then nail them together. You could even add legs or wheels to add storage space underneath.