They are designed so the bear can't pick it up or get their teeth around any part of it. Tying anything to it defeats the purpose. They Work very well in yosemite, where the bears are ninja food getters.

They can be broken by very determined bears (i have seen pics of a shattered bearikade) but that is a rare thing. Most of the time the bears roll it around and move on. It does not matter if the bear can reach it, smell it, or touch it. It just keeps them from the food long enough for them to be frustrated enough to find easier calories.

In yosemite we would bury the cans in rocks so we would hear them and have a chance to drive the bear away before he rolled it off somewhere. Tree wells or large depressions where a tree fell and left a hole are what i put mine in.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki