I don't think I've tried anything like that, but a couple of suggestions -
If you're not afraid of math, I'd try the pythagorean theorem - x^2 + y^2 = z^2, where x and y are the "lengths" of the known sides (perpendicular to each other) and z is the hypotenuse (I'm assuming your dimension of interest)... you may need also need some graph paper to make sure you're getting 90 degree angles.
Another (possibly) more convenient method is to cut out your top and right sides and connect them with sheet styrene or wood to match the dimensions, allowing the corners to extend into where your angled piece is supposed to be. Then sand down the corners and lay on your armor.