The tip about the inside dimensions is the soundest one I've seen so far, as all hives differ slightly on the outside. Nothing wrong with a ply brood box, but ply supers get very heavy. Marine ply is very expensive and WBP (water bonded polymer) is equally good for the purpose. Don't, don't don't use Pine (Scandinavian Redwood) or white wood and do not buy it from a DIY warehouse. You will get boards bigger than 3" (75mm). I buy Western Red Cedar boards of 1" (25mm) and 1.5" (38mm) thickness from a timber yard.The timber is not expensive, but if you then have to pay for it to be machined - - .
If you really want to be confused, or want to stick to one set of plans, I drew up the ones that a certain Book company sells at shows and online. You can have a set if you let me have your email address.