I just finished filling at 6ft Fairtex banana bag not long ago.
Mine is filled with about 50% rubber carpet underpad, cut up into small pieces....and 50% clothing, sheets, etc... cut up in to small pieces.
I then bought some new carpet underpad to line the bag with (about 1/2" thick) so it would keep a nice shape.
All told, I think between my wife and I we spent about 10-15 hours cutting over a few weeks, but is wasn't that big a chore really when done in small doses while watching TV or a movie. There was no rush for me as I was waiting for the bag to come from Thailand anyway.
The whole thing was a bit of an odyssey, but I'm very happy with the result. The bag is hard enough to condition, but not so hard that it discourages me from kicking it full force.
I'll post picks of my home gym setup one of these days in the near future.