Will this work and do I need a fuse? Mean Well power supply

You could easily put the shunt behind the battery box with just the addition of one fat cable from the negative post to the shunt, then attaching the other negative cables to the other bolt on the shunt.

You may have a perfectly clean and acid free battery compartment, but it doesn't take much to have gassing that can corrode your connections and any exposed metals. I wouldn't put my shunt in a battery compartment unless there were no other reasonable option.