heres some pics of when i built the stand for my 180.... its a lee mar flat polished glass tank so its heavy ....
anyways i used 2x4's and doubled them up on the legs i ran 2x4's for the top and bottom fram and then cross braced with 2x4's and added metal L braces and such....
for the top layer i used 3/4" plywood and same with the bottom part of the stand that holds the sump equipment....
i then sealed all the wood with a white primer... to make sure it doesnt warp or anything.....
then i added a 3/4" high density foam sheet on top of the stand to help ensure a flat level pressure point free surface for the bottom of my tank....
hope that helps heres some pics....