For the reach undersuit, it looks like it is pretty simple. You can just use a black morph suit and it will get the job done because of how much of it is covered by the armor. The halo 4/5 suits, not so much. If you want to add detail to the stomach and back, you can use craft foam or some layers sewn ontop of the fabric. One of my favorite reach undersuits is shown of in
KitKatGoose's thread, and she nailed it, while making it comfortable. Sewing with elastic so it can stretch, and it is really simple.
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