I would use 3/4'' just for the fact that it would be easier to fasten the box together. Plywood is a really strong material and 1/2'' would hold up just fine, but try screwing a 2'' wood screw into the edge of 1/2'' plywood....it will usually split or break. With the 3/4'' you actually have something to screw into (even though it still may split). I did build a 4' x 2' x 1' box out of 1/2'' plywood that held about 200 lbs of steel without a problem. If you decide to go with 1/2'', you might want to support the joints with a ripped in half 2x4 so that you will have something to screw into. That would work just fine.