You'll have to make your codpiece out of a different material if you're doing papercraft or what have you. I saw a stress test vid. from 405th.com and it said to make it out of urethane foam because it's flexible/durable and molds details well. If you're still interested, I can link you.
Wow that was a long time ago.actualy I made another suit of armor since then and I just put two layers of fiberglass on the cod as well as the thighs then did a little trimming so it fits right ,it holds up well when I sit down and its fairly comfy.thanks anyway though.