I got the alum battery Box from Quick Cable Corp, I'm a Dealer for their products through my Battery Shop in Northern Calif...the # 120320, unsealed box that I used cost about $130.
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I put a Group 27 Deep cycle battery in the box to match the two other GP-27's in the AS forward boxes. I used AWG #4 battery cable for the short 'run' from the new box into the curb-side AS battery box...I was able to run the cables through one of the lower vent holes in the AS box door, then connect to the battery inside via a Quick Disconnect connector.
I used some one inch wide SS 'bands' to hang the battery box to the frame rails...the SS I use was a bit too thin, in that the box 'moves' around somewhat...I'm going to have to use those bands as a template to bend up some thicker material to firm up the mounting.
Here's some closer pic's...