I would go for a stretchy fabric that you can make a bit smaller so it hugs your figure better. And then for the stiffer parts around the 'skirt' area you could sew in some backing so it's not all floppy. Or even glue the fabric to craft foam so it's stiff like the rest of the armor but can still match the upper part of the outfit.
Stiff material will never hug your figure like that unless you get it extremely well tailored down to every exact measurement of your body. Thus, it's better to go with a stretchy fabric. Make sure it's not see through when it's stretched out though. It wouldn't be bad for the stomach area since it seems to show her stomach nicely but I'm sure you don't want your boobs/bra showing to the rest of the world.