First, go to your local lumber store, or to a construction site where you can beg for some scraps of 1/2" plywood. My friend Betsy actually found the wood for me--and talked the construction worker into cutting the wood the right sizes for me! Of course, Betsy could talk the hind leg off a mule, so I am sure this was no major feat for her. Of course, if you need to have all the measurements calculated before hand. Remember to include the 1/2" thickness in your calculations.
The sides were 4" x 12" and the front piece was 12" x 28," so the top was 4 1/2" x 28". I used Liquid Nails in my calking gun to adhere the sides to the front piece before I used finish nails to secure it. I did the same with the top piece. (I tried screws, but they started to split the plywood, so I opted for the nails instead.)