I was looking into making a Karn for someone and had to ponder this question. While I didn't actually implement it, I was going to go down the bodysuit-and-foam plan. I was thinking that heavy-duty EVA foam people use to make iron man and mass effect body armor. The bodysuit underneath would be dyed roughly the same color for the flexing parts between rigid armor sections.
Here is just one link to an instructables page on crafting armor out of EVA foam:
How does that sound?