It is not hard to get good tone - it just has to be done right. I built some gtr cabs. The first one I built of plywood from a lumber yard. It sounded dull and thuddy. My buddy recommended void-free birch plywood at 2X the cost. Built one from that and it was fan-damn-tastic!
Construction technique is the hard part, and you've got that covered. Calculate your dimensions carefully to construct a "sound" that you want. Only you can decide that.
OK, maybe my reply isn't too helpful, but I just wanted to point out that it can be fairly easy to do it right if you've got the bases covered.