I've run with Alaskans who use a firearm as bear spray. Their method was to have the non-running partner pace the one training on a bike, carrying the extra weight. I don't remember which one they chose, but the one that had enough stopping power for their local hazards, was too much extra weight to run with regularly.
Repeatability does sound like a critical piece, especially when it is so seldom used.
My thought was the can having some kind of cover or 'glove', which itself mates with a belt or receiver on the person, also leaving room to carry it in hand (hand strap?) for short jaunts (5k run or small hike) or set it on a table in camp.
A great place to wear it would be on the lower back, sideways, to be slid out with the dominant hand. (right in both our cases). This would allow the person running behind similarly easy access. That's likely the best place for running, minimizing bouncing would constrain it a bit further, perhaps fixing it at both ends of the can.
While I can sometimes outrun my sweetheart, I hope my stubbornness prevents such. I know it prevents inviting others along on some of our romantic forest runs. Bringing along a 'bait-runner' isn't always practical, so I'll have to do my best with planning this.
We run with belts already, for the dogs leash, phone, water bottle and whatnot, and the belt area can get busy. We sometimes run with hydration vests, but that is only for longer efforts of an hour+
We also have a teeny 'coyote spray' for in town. We just started seeing coyotes up this way, and I've been followed by one, for about a km before I decided to run it off. It never got close enough to spray, but if I was shorter, it might have been braver.
Maybe I should start with that, as a practice round. I feel my skills are a bit below 'rudimentary' at times. Design, by the way, is about the most daunting part for me.
Found a way to insert the image...