The temporary cord lock rapidly converged on this panel thing. It's something we've no doubt seen forever in food packaging but never remembered. Despite its simplicity, it grips better than spring loaded ones. It works better if the elephant foot side faces away from the tension.

Note: they work better when the string is pulled opposite of the opening, so this design would be best as a O with the 2 openings on the inside of the O.

The mane problem is it doesn't manage the excess string. There could be a provision for tying 1 end off & letting the other end move. 1 end could be taped together.  When there's lots of excess string, it's traditionally been tied into a loop.