If you are still wondering about the middle brace, it should be pretty straight forward, just run a even/thinish (depending on glass thickess) bead of silicone, same stuff you seal it with, allong the top edge of the glass sides (make sure its clean and smoothish mite be good to clean with acitone so as to make sure its grease and grit free) then drop your center brace on that and tape it in place over night. Here is a link to some DIY aquarium stuff if you doubt my silicone suggestion lol
http://saltaquarium.about.com/od/diytanksrefugiums/ss/diyaquarium.htm as all glass aquariums (as far as i know) are held together solely by silicone, except once you start getting into the huge sizes lol.