Since my posts above I have put in golf cart batteries which required raising the box height a bit and re locating my wires. I now have photos to show what I have done.
The first photo shows the top of the batteries with the shunt on the far left side, laying sideways in the negative side of the wiring. As I mentioned before, I used a piece of flattened copper pipe, with a twist and holes drilled into it to both do the electrical attachment and the physical mounting. BTW, the center two large wires between terminals are the old red and black AS wires to strap the 12 volt
batteries together. I simply re used them as the connection between the two 6 volt batteries. A small fuse shows (blue wires), it is for the power to the tri metric meter. The other fuse goes to the power jack. Also note that there are two large red wires (+) and two large black wires (-) as my rig has the inverter option which has it's own wires directly tied to the batteries.
The second photo shows the shunt up closer.
The third photo shows how I raised the top of the battery box. I used 1 1/2 inch c channel aluminum and the original box lid.
The forth photo shows the blue plastic I lay over the top of the batteries, as a just in case situation to guard against shorts from the aluminum box top as well as some water and dirt protection.