I didn't see the episode, but I can't imagine mdf covering plywood. It was probably MDO, which is sign board. Common is 1/2" plywood and it has covering of (I think) paper that is coated, etc. You can buy it with one or two good sides. In my opinion it will still rot eventually. For signs I use 3/4" partially expanded pvc foam sheets that have a nice paintable finish. This stuff is called Sintra (trade name) and you can get it at a good plastic distributor, I use cadillac plastics. It comes in various colors and between 1/8" to 3/4 and maybe thicker. It is stable, and can easily be cut and tooled with wood blades, etc.