It took me 165 spawns to get 3 casks. I saw 20 gnoll brewers, 145 gnoll guardsman, and 0 master brewers. I also killed about 10 spawns before I started keeping track, which were all guardsmen. I saw brewers on spawn number 4, 7, 8, 29, 38, 49, 51, 55 (cask), 69, 92, 93 (cask), 109, 134, 138, 142, 145, 146, 147, 161, and 165 (cask). Spawn rates are between 13.5 minutes and 17.5 minutes usually. And yes, the brewer/guard spawns around -32,-32 and then wanders over to the barrels after maybe 10 minutes. They do eventually wander back to the water as well. I also posted this information on the quest link.
As for the item itself, it is a container but these days you can put them inside a backpack.
I finally got my Cloak of Jaggedpine at level 49 and I wear it to show my allegiance.