I bought a Spiral metal tube but this appears hopeless. I cannot fit it in. Too much in the way. The original is some kind of heavy paper material which was rotted. They dumped concrete on top of the pipe which caused the leak then planted the duct in the concrete. I have the leak fixed and now need to repair the duct somehow. If I could find something flexible that could also support the concrete that has to go on top when I repair the slab. Even if I find something flexible I don't know how to see the ends. Perhaps some kind of spray foam around the ends but no clue at this point. This duct feeds three beds and two baths so I don't want to downsize the gap filler piece if possible. A 10" piece could wiggle between the gaps probably.

http://www.youtube.com/watch?v=1VLqPmLRi9s