If you are going to get the standard sized frames, you may as well buy a standard size tank .... it will be cheaper & easier.
DIY tanks are only worthwhile for odd sized/shaped tanks for specialized locations, or for really big tanks. <A large acrylic tanks is cheaper to DIY than pre-made.> For the DIY tanks, people usually use a "Euro-brace" for the frame & cross brace. This is a rim of glass (or acrylic) that you custom build to fit inside the tank. Essentially, it is a stripe of glass a few inches wide that you silicone to the perimeter of the tank top (at right angle to the tank "walls"). The cross brace is a piece of glass (~ 6-8" wide for larger tanks) that you silicone across the top, joining the front & back panes. <You don't need to do a rim at the bottom, as the bottom pane will act as a brace.>
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.