Since budget was a big factor, I used sanded B/C construction grade plywood. It's basically a little nicer grade of house sheathing. Although it's great to work with beautiful, pristine materials, I also love the challenge of making something beautiful with utilitarian materials.
The corners are mitered, glued and cross nailed with masonry nails, which have a thick, irregular head. The heads are set just on the surface, not driven home. Sorta like a row of rivets on a steel bridge. It's not traditional joinery, but it's strong and interesting and quick. Fits in with the whole industrial vibe. The feet are blocks cut from an LVL beam, so they match the plywood edge perfectly.