So, you're making the final armor then. Good choice.
Anyway, I managed to find a grainy but larger cel photo of the final armor.
Here, is however, a fanart picture that shows everything in better detail, but which I think distorts a couple details.
I recommend, that you use multiple layers of heavy interfacing and a slightly curved seam running right down the middle of the pauldrons to force them to maintain that shape (if you have trouble picturing that, examine the use of seams on a bra cup), and then attach them to the costume using snaps so they can be removed.
Now, if I was building it though, I'd throw out the whole monopiece costume idea in favor of something more grandiose.
The torso would be spit into two components. A combined skirt/shirt (for lack of better term), an upper torso breastplate, and the pauldrons.
The skirt/shirt would be everything that is black in the picture, and down; it would have as pictured a mandarin collar in black with gold trim. It goes from black to red to white to gold and then ends. But the red on upper torso would be a separate 2 plate piece that you wear over that garment. This would be made of cuir bouilli
or just good olde 22 gauge steel. Whatever you use for the plate, you also use for the pauldrons. and you just have the whole thing be connected together with rings at the shoulders, so it splits apart just under the arms.
It wouldn't be totally correct with the artwork, but not totally incorrect either. The dimensions of the final armors aren't very easy to make out in canon art because, well, canon for Rayearth was long, long ago (animation being what it was back then...). The dimensions suggest that Hikaru is wearing two layers starting from the mid torso band of red up to where it ends before the collar. It LOOKS like a second collar, which would be the case if she were wearing plate over another garment.