I have the one gallon rotopax gas can. I mounted it flat to my rear cyclerack. My Dad bought a Kolpin but after I took a look at it, the shape wouldn't allow it to lay flat on the rack. I found rotopax online and ordered one for both of us. They are more expensive, but they sure are a stout can! They are rock hard. The shape is more rectangular and was a better fit for me. You should be able to stack them with one of their extended mounts.