Great Little Cash Stash Pouch
I have been using PacSafe RFID blocking waist wallets for quite some time to store the cash I travel with inside my carryon and check in luggage. You can sew the waist wallet into a discreet interior pocket in your carry on, which makes it a lot less likely that the cash you are carrying will somehow turn up "missing" when you go through security at the airport. (Even if it shows up on the bag X-Ray, it will be harder for someone to rifle through your belongings to get to wallet, especially if you have sewn it into a very interior portion of your bag.)