Go to the hardware store, and look at what they have there. Ikea is another place to look, if you have one. All that you need, is a drawing of the dimensions, take that to Lowes, Ace Hardware, Menard's, and they can cut a sheet of plywood, or pine board or such for you. Take that home, and paint or stain and put together with some screws. keep in mind, that you will need a drill and some drill bits, so you can put in the screws.
Even better, look and see if you have any mill or cabinet shops in your area, they may have something, or make something for you.