Add third 12V battery in parallel to travel trailer?

I agree, that's a terrific resource!!

I only need to fabricate a battery rack to hold my third battery now. I have two that are next to one another length wise. I have enough room to install 3 if I put them sideways and move them back 6". I may use 3/4" plywood or possibly fabricate something from angle iron.
Anyone know if there is something off the shelf that will work for me?

Always a fun project!

Scott