Thanks everyone for your wonderful ideas and links.
Alas, I was not able to come up with an armadillo costume. I did call some costume places and Ben and I went to one today in the hopes that they would help us piece something together that could pass as an armadillo.
No luck. We were told that that unusual-looking creature would be a "custom-build" and would require lots of advance preparation.
So, instead, Ben has decided to be a musketeer. He has one of those cool hats (see above) and a burgundy cape, some red gloves and a flintlock pistol replica (I think that's what it's called).