New Day Tank

Started making the new Day Tank today, the picture above has turned out rather un-square, must be the way I stand? With having a riveted bottom tank, I though it best to have a riveted day tank to match and, before you ask, yes they are real rivets. However, due to not having a Press Brake big enough to fold 4mm plate I have cheated and welded the corners and fabricated the bottom tray....and....welded the inside. So, really, it just looks riveted.
I've the top to make yet and that will be angle iron riveted inside, yes proper riveted this time, with the top plate bolted inside that to form a tray. The filler is to be a 3in hole with about 3in combing round it.
I don't know what to do about a level gauge just yet, I'll have to have look round to see what others do. It'll probably be somthing on the lines of a boiler gauge frame I guess, I'm open to suggestions.