Disappointment with spring loaded cord locks got lions thinking more about a friction based cord lock.  Previous ideas centered on passing string through a TPU cylinder that was welded closed.  It would provide enough compression to keep the string in place while still allowing it to move with enough force.

The problem with spring loaded cord locks is they don't provide enough grip.  They might as well not have a spring for the amount of grip.  It's normally not necessary to release the spring.

With a friction system, it wouldn't be possible to take the string out & reinsert it.  For these temporary applications, there could be a larger device.  The smallest device would be for clothing & sandals, so it could possibly be permanent.  The temporary design could be a larger thing which permanently attached to 1 end of the string & created temporary friction in the other end of the string.