I'm embarking on making my own too. Trouble is, good dressed lumber is tough to come by. I have to wait to make a trip home to find some. I'm going to use 1.75" oak or (depending on price) honduras mahogany. Laminate, thickness plane, band saw, sand nice and smooth. A heavy, hard wood will be rock solid. Those platforms take a beating so the thicker and heavier the better.

The bigger the bag the slower it is, and the easier it is to learn with. But I say just get a smaller one to begin with because you'll very quickly learn and out-grow it. ie, it'll very quickly be too slow for you.