As with anything you build, the better quality the materials are that you start with, the better the finished quality will be. Use no less than half inch thick plywood, but I would personally pop a little extra for the 3/4 inch heavy duty plywood. I used treated wood for my frame work, prevents rot. You can get away with 2x4 framing, but a little extra and you can use 2x6 or even 4x4 on the corners. The more you put into it, the better the end result will be.