I used an inexpensive motocross vest for the undersuit. It made attaching the armor easier, plus the back protection looks like the back of Master Chief's suit.
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The mesh also allowed for more airflow, and I used a black drifit long sleeve top with black leggings for underneath.
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The arms stayed on with friction and some velcro pieces attached to the undersuit.
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For the gloves I used some green armored paintball gloves that I glued on the 3D printed hand plate to. Here you can also see the foam thigh piece. I used an elastic strap that looped around the belt that I used for the codpiece to help keep the leg armor up.