Your inbox is full so I am posting this to you here.
I used the aluminum trays from BassCat. In order to get 3 group 31 batteries in an RT188, the trays must be thin width and line up side by side with the holes for screws inside the rim of the tray. Here they are, they are not cheap. https://www.basscatstore.com/aluminumtraywithstrap.aspxOriginally Posted by Bassakwards
I cut a sheet of 3/4" birch plywood and sealed it with sanding sealer. It was cut to run the full width of the battery compartment and a little wider than the battery trays. Don't use treated or Marine plywood. The chemicals in it can eat away at aluminum. Once the plywood was screwed down to the braces and battery floor sheet, it makes it very easy to line up the battery trays and gives a good surface to screw them down to.
I used Exide AGM XMC-31 batteries because they are dual purpose and have both the reserve capacity and cranking amps I want. I also have a local dealer that orders them for me fresh direct from the factory warehouse. But I am familiar with the batteries you are talking abut and think either would be fine.