noob looking for pointers for making square boxes

[necron99]

So I am building my first paper model - a tank from Warhammer 40k called a Baneblade. Found some great plans online for it. After I started looking at various cardboard sources I have lying around the house I think cereal boxes work the best for the best thickness that works with matching the shapes (I tried 1mm thick board and found it offset too much).

So my latest dilemma seems to be how to build a perfectly square 'box'. By that I mean any hollow object that is basically rectangular in shape. So far they're all off by the just a little bit which adds up by the time you've glued one box onto another followed by another. Is there some jig or process that I should adopt that will produce squared boxes?

Thanx!