It's funny how history repeats itself. During World War II the British were especially clever about giving their troops ways to hide various escape equipment on their persons - and one of those was the "boot heel escape kit," which could hold (I believe) a hacksaw blade, compass, cord, needle, thread, etc. There is an excellent discussion of this in the book "The War Magician," which details the exploits of state magician Jasper Maskelyne. Amazon.com Link to the book on Amazon.com