I just looked up "dog spray" online to see the shape. Looks like your typical spray can. I'm wondering about that company called Built that makes neoprene cases for all sorts of things. You could zip tie something made of neoprene (you might have to cut and custom stitch something for yourself) to the bike stem and slide the dog spray into it.
Or...here's the real creative part of me speaking. Get a toilet paper roll, cut it to the correct height, waterproof it with duct tape all around (inside & out), zip tie it to your stem and your dog spray should slide right in.
Not very fashionable for your pretty bike, but it would do the trick.