I have always built my benches with a 2x4 frame on 4x4 legs with 1"plywood for a top and a loose piece of masonite on top of the plywood. The frame is secured to one or two walls. (my current bench is L shaped and in a corner) I don't attach the masonite to the plywood other than were the press, lubesizer, and other tooling are mounted to the bench. The idea behind this is if the masonite gets messed up I can replace it without having to remove any glue from underneath and there are no nail or screw heads for things to get caught in or on. It gives you a very smooth,easy to clean,level surface. For the top plywood either 1/2" doubled up or 1" will work fine its been my experience the thicker the better and to plan out your placement of your tooling before you install it on the bench. Nothing gets your attention faster than turning from your scale or powder measure and cracking your funny bone on the reloading press.