To sort of follow-up on Jim Keenan's suggestion, I've long used a couple of *thick * cardboard tubes (from the inside of that old roll-type fax paper)in a wide array of places that frown on weapon carry. (I wish I could talk more about it, but I don't want to mess up a good thing.) No one has ever looked at it long enough to determine that it's *much* thicker than a paper towel tube -- the walls are 3.5 mm thick. I've had to use one once, and it was very effective (leaving a brutal circular cut in a dazed forehead, and G-d knows what squished in a throat). I had my godson draw on the outside of one in magic marker, and when anyone gives it a second glance I just tell him it's some artwork my kid made for me that morning. Never had a problem. It weighs 1.4 oz. so it rides happily unnoticed in the inside breast pocket of a suit or blazer, and the cardboard provides a remarkably nonslip grip.
I've brought screwdrivers (in the commercial packaging) on planes before, but you can't get away with that in some of the other places I have to go. Mr. Cardboard tube has never failed me.
Cheers!