Yeah, My first thought was to build a tray behind the dash. However in my situation I now decided against it. Reason - We built a custom dash with a steering wheel that is going through the dash. However the dash is the support the the column. So If I put the batteries behind the dash I would have to remove the entire dash if I need to get to them.

I think I have now decided that I am going to mount two batteries behind my front seats in front of the fuel cell. That way if I need to get to them, I can just remove the passenger rci seat (attached to cage).