My bank is four group 31's in parallel - I thought about using buss bar or hammered flat 3/4" or 1" copper pipe, but decided against it. Seeing how things move around in my RV while it's bumbling down the road, I figured the #0000 cables were stiff enough and that a ridged connection between the batteries would be asking for trouble. they might not break anything I could see, but what if it placed stress on the post to plate connections inside the battery?
It sure would make it cleaner looking, but looks aren't everything - especially since I don't have to look in there that often, just every six months or so to check the torques on everything.