I ended up making the bottom sheet the base, because that way I don't need to get a nice clean cut (which worked out well, since I had to cut off about .5" from the sheet, and the edge ended up a bit jagged). The dimensions are 12"x10"x9.75"
Always build on the bottom glass not around the reason being that building on the bottom ensures that your tank will sit perfectly level. Building around could leave you with an uneven bottom.