I'll add that my box beams are 1/2" ply. I screwed into predrilled holes in the sides w/o glue with no problems (I was concerned like Rehd, but it worked OK). 3/4 ply should be even better. One thing you should add are short pieces of wood (~2' or more) which overlap the butt joints on the inside. This will substaintially strengthen the joints and entire structure. As far as the layout, that's pretty much what I did.

BTW: Nice drawing!

Bill