For some reason, I thought about this when I read "shoulder straps":
Instead of being attached to another person, imagine the strap being cut in half and attached under the bass via the endpin or a D-ring of some kind on the lower bout.
I get a little nervous using backpack type straps. With this idea, you'd be holding the bass in front of you, but most of the weight would be bourne by your shoulders. Just a thought