You can make handcuff shims from anything that is the right thickness and which is stiff enough to do the job. One such material is the plastic strapping that freight companies use to secure their loads and packages. The plastic can be cut down to the right length and width using scissors. The banding can also be torn along its length. If you can’t easily tear it, then try bending the end(s) back and forth to make a weak spot. Being plastic, it is immune to metal detectors.

The following images show how shims are used to defeat the ratchet mechanism of single locked handcuffs.

Note: You may need to apply forwards pressure on the shim while pulling on the arm to release the cuffs.

Also note that anything that is narrow enough, thin enough, and strong enough can be used as a shim. A good example is a bobby pin, as shown here: