One solution I have (reluctantly) used is to install high-temperature sleeving.
It is manufactured as electrical insulation and comes in a variety of materials and temperature ratings.
I read about it in a book, and decided that, in an emergency, it would be acceptable.
The other solution is a "throw away box..."
Pry out the existing box, splice a short piece of new cable, and push the old box aside (throw it away) to make room for the new one.
