Someone above noted that an elastic net would probably hold the canister on your rear rack, Djb suggested straps. I would try the elastic net. I was surprised how well it worked on one of my dry bags.
In my case above, the rack is a Logo which has a very narrow platform, but the two panniers were higher than the rack and that made a nice place to put the dry bag.
Originally Posted by
gauvins
Really impressed by your throwing accuracy. (great pic)
Lots of practice over the decades. Some people put the line in a weighted throwing bag, you toss it and as the bag sails through the air, the line comes out of the sack. But I have never found the right combination of sack and line to work well.