Bevendre

I’ve not heard of this fanon idea, but I rather like it.  Assuming that the holes on a changeling’s leg are unique to the individual then having a locking system reliant upon turning a series of pegs in a pattern fitting the holes on a changeling’s leg, and dependent upon variable pressure could work.  The sequential locking system would be similar to the common setup of locks in which the tumblers are fit in place and opened by the teeth of a key, and a pressure system like that which exists in some safes to prevent them from being broken into.