The solution above sounds like a good one.

The only other suggestion I have (if you can’t find a connector like the one in the answer above) is to connect the shunt as previously suggested (2 wires from the service battery negative to one side and 2 wires from the other side to the engine start battery negative) and then connect your ‘fat’ wire and the 6 other negative connections to the engine battery negative post. This point (engine battery negative) becomes the common ground or negative for the whole boat and the shunt will measure all the current drawn from the service battery only.

I would commend the separate bus bar/connector approach, though, if you can find one.