Boot Lace Handcuff Key
The key attaches to the end of your bootlace as shown above. Its very secure and no need to worry about losing it, even if running. I used it on my desert tan combat boots so that you would see the key by way of contrast. If used with black laces it would even be more discreet. It could also be painted to match your desired laces.
The key is metallic making durability of the product greater. Being that most shoes have metal eyelets or other metallic feature; I don't feel it would be discovered under metal detection.
The kit also comes with a rubber outer cover to protect the key and keep it hidden. This item retails for $15 and is available from Shomer-Tec.