Was surprised to read this thread, because I’ve used ursacks with opsaks for caches a number of times and they’ve remained untouched. With a couple qualifications: It was an earlier ursack material, a heavy green Spectra no longer offered; and clips were placed on the opsaks inside that sealed each of them airtight. Also, the lines were thrown over branches that were at least twice as high as a large bear’s standing height, and not sturdy enough to support a bear that climbed the tree and tried to go out on the branch. The line from the branch was wrapped around the tree several times before being secured lower down, so as not to offer a bear an obvious grab. The bags were hung well off-trail, so much so that I had to draw maps to find them again. They were left for periods running from 7 to 10 days.
Agree that once the bear has a whiff of the food, the game is probably over. The bear will find some way to get the bag down and open it. So the clips on the opsaks were probably key. The double clipped opsaks were also tested in the forest at home, hung in common nylon stuffsacks, together with the same bait hung in a stuffsacks about 20 feet away. The stuffsacks without the opsaks always got torn down, and the ones with opsaks protecting the food remained untouched. The results were posted on BPL a couple times. Also tested a cheaper competitor to opsak, and it did not work – the stuffsack got torn down.
The clips were called “Clip-n-Seal Bag Clips – Large 10 pack,” and are made of a white plastic broken tube about 12″ long that clips over a smaller tube about 2″ longer so they can be easily unclipped. Recall they came from Amazon, but not sure. The tubes can be cut to fit just over the widths of the opsak openings.
Went to all this trouble, because did not want to find my caches invaded after a week’s trekking, with no food and some other small essentials for the next week. A friend had that experience, but with a human invader, and saw first hand how much of a bummer it is to have to abandon a long distance trek when a cache is invaded. But she was really frosted that the invader took her small bottle of hooch.
Note that in use of ursacks for camping, I use the kevlar cloth ones, and just hang them up in the same manner well away from the tent site. And avoid tenting in areas with signs of use by other backpackers or bears, especially berry patches. Note that this applies to black, not grizzly bears. Met the latter early on in the Canada rockies, and have stayed away from griz country ever since.
Had a problem only once when forgot a bottle of blueberry Kool-Aid was left in a corner of the vestibule. Spent the night with a candle lantern hung in the front of the tent, pepper sprayed around the tent, and banging pots. The bear kept me up all night and did not leave until dawn. Could have just thrown the bottle out of the tent for the bear, but did not remember it was there. Still wonder if the reason Kool-Aid withdrew the blueberry flavor from the market after that was because some poor hiker got injured by a bear and sued.