What if there aren't any trees nearby?
If possible, use a rock-climbing aid wedged into a crack high up on a rock, then tie URSACK to the aid.
Since bears can't climb rocks, this should be very effective.
Otherwise, tie URSACK to any object that is too heavy for a bear to drag away.
Yes, we know there are no logs like this above tree line. Find a rock that looks like a log or . . .