I would regurgitate what was said in the link. I did mostly as he said. If you have questions, email me for details:
Summary: 3 compartments wide (8 13/16 in) by 10 compartments long (8 7/8 in)
BIG heavy box, absolutly no twist/warp ( < 0.1 deg ) and only a couple of high spots that can be sanded down, on the order of 4 to 6 thousands of an inch (thickness of copy paper), according to the 6 ft level. Have set it aside, on edge, until needed. Now to finish the tanks.