Am I being rude?

I've been seeing this guy for a week and he asked me to a costume party. The problem is he told me late last night that he wants me to be Imperator Furiosa. I literally can't pull that off in a day, that arm is horrible. Is it rude to be honest and tell him I cannot pull that costume off in so little time? I just got an apt and don't have a bunch of extra money trying to put it together.