I've used the sand fill before. There are a few essential steps. First, cap one end, then fill the tube all the way. Place it on a vibrating plate so the sand settles in, fill it again until you can just place a cork at the other end, then go bend.
Other approach is various formulas of low melting metal.
See one at:
http://www.csalloys.com/cerro-alloys...ng-filler.html