This GoPro harness mount, as seen on Reddit looks like a more stable version of the impromptu dog mount that was featured here earlier. As you might be able to tell from the picture, the base is the ever-useful square mount that comes with the packaging.
The packaging mounting square (see this post for how to get this “bonus” fixture) is modified to fit the
harness camera saddle, a “premium touring harness” from a company named “Trixie.” If you have a hard time finding this particular model*, I would think many heavy harnesses could be modified to use this kind of mount.
Check out the video after the “read more” to see the dog in action “interacting” with another of its kind. As any dog owner knows, politeness varies depending on your species…
Here’s the link listed for the Trixie harness product. It looks like it takes you to their product catalog page, not directly to the harness. I didn’t want to link it without some explanation.