Cool.... some people use PVC piping or 1/4" particle board, and others use foamboard, cardboard and plasticard.
Personally, I'd use large hard foam blocks to build the basic shapes of the torso, head, lets and weapons, and then use PVC as a skeleton to keep it all together. Cardstock and plasticard would then layer over the entire mess to get the actual model armoured up.
Good luck with your project!