There is a super small one that is fairly cheap ($39) at http://www.bareboxer.com/
It fits in just about any size pack.
I just bought one and it is roomy enough for a day or two. and it weighs less than 2 lbs (Yeah it is still heavy)
Anyhow it ships pretty fast since it is coming from California.
The weight is slightly offset by the advantage of conveniance..
I have an Ursack and it works well for rodents but I think it would make a chew toy for a bear. I am sure he wouldnt get the food but he would definately be able to chew and chew and make the bag pretty uh chewed.