How much you willing to spend to make it usable. Cheap means towels and cloth diapers. I did this with all of my packs until I bought a dedicated camera back pack. Never broke or scratched a thing. Kind of messy on site because of all the wrappings. If I was going that route again I would chunk down money on some fly fishing reel packs and other paraphernalia.
Just start fiddling with combinations and realize that all that crap gets a LOT heavier the further you walk, and you have to walk the same distance back. Figure out what you HAVE to have and keep it minimal. once you trim down the trimmings there will be much more space in the bag to pack padding into.
By the way well washed cotton cloth diapers totally rock as lens wraps.
Wyno's ideas sound real good.