please make more of this this. i found this tutorial is the best I've found on the internet. it can be applied to any type of cos-play or armor construction. also i would love it if you could post more of your rough work, i found your sketches proportioned well.
It's almost impossible to make something perfectly round with purely cardboard. To make it rounder you need more "segments" and you have to make it rounded by applying something, for example a car-body-filler like Bondo and sand it.
To cushion it I just going to glue some foam on the inside.
No problem, at first I wasn't a big fan of Bondo. It requires a lot more work but the result with it is great. Your prop gets a lot stronger and you get a solid finish so I really start to appreciate it.
What about a light to medium type battle armor heavily inspired on stealth bombers. Because stealth bombers have these triangular shapes, which is easy to make with cardboard. This way it's an easy step into making armor, I even would be willing to make a design.
Symmetry is really hard, the helmet in the tutorial still isn't perfectly symmetrical but it looks good allright and the approach works pretty well. The building material is cardboard, it's cheap and easy to work with.