I was going to make a harness mount but found one online made by K4, It's made well and cost about $110 with the exchange rate at the time. I mounted it to an old harness I had, Just as a quick test as I wasn't ready to drill holes in my brand new Dakine harness at this point. The old style harness was very soft and flexy, I used one of the mounting methods (1 of 2) that had the least secure fixture to the harness and this did caused a fair bit of bounce as can be seen in the video but it still did an ok job. Later I mounted it a bit more securely and this did stiffen it up quite a bit which should stabilize the image a fair bit more. I'll continue to use this harness until I find the best spot for stability then I'll fix it to my new harness, It's a lot stiffer and should fix the issue that the softer harness has. The pole can be removed quickly for when you don't want to use it, The base plate will stay fixed in place with no ill effect. There is a safety line for the GoPro that's attached to the base plate, This can be rolled up and stuffed inside the mount. I did have to be careful when water starting, The first few times I whacked the camera but it's not much of an issue. I'll continue to test it this week and I'll update you guys when I know more.

I bought one of these too - a well made piece of equipment. Used it recently at Gnaraloo. Check out some of the stills.

I mounted the backing plate by cutting a slit in the harness and putting it inside the material, rather than bolting it straight onto part that faces your back. Not sure my way provides the best stability - my guess is that mounting it straight onto the part that faces your back will be better.

I'd be interested to hear more about what you've tried out and what seems to work best.

Wow they're pretty good digger.