Help With Grow Box Design Please?

So I need to build a 4'x6'x2' closet grow box with 2"x2"s, however I want to make sure I do it right, measure twice cut once blah blah blah. All the grow box build vids Ive seen show them building the sides first and then connecting them together, however my friend suggested I build the top/bottom first, connect them together and then add corner supports, I attached a paint sketch of his suggestion, however will I even need corner supports with a box this small? For lighting I will be using a 6" air cooled hood/reflector (HTG's Euro Reflector) with a 400W CMH + 250W HPS, this will be vented with a 6" inline fan. So if someone could suggest a simple and efficient grow box design for my situation I would really appreciate it. Thanks for your time.