While it’s designed for dogs with mobility issues, the Help 'Em Up Harness is supposed to be okay to leave on. Their blurb:
What makes this dog harness truly unique is it’s designed to stay on your dog for extended periods of time. Unlike towels, slings or leashes that need to be rigged with each use, or ramps that have to be moved or adjusted, our harness is right at hand, whenever you need it.
http://helpemup.com/features/
I have one for my sometimes shaky old dog, and it is a very good, well-designed product. I’ve used it, but haven’t really tested leaving it on for longer than a walk.