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A deer drag works and costs about ten bucks.
If I use anything besides the rope, it is just the waist belt from an old backpack. I don't see benefit in a harness you need to strap over your shoulders- it just moves the force up higher on your back and pulls you over backwards- you have to lean forward more to counteract it. For dragging a deer, I suppose it would be good to pull up on the thing, since it does not slide that well, but with a sled, I think you would want the force pointed more forward, and less up.