Hello, I have a 35GS 2006, so a little different. I did though add two additional batteries in the compartment directly beside where the factory house batteries were. I built a shelf, and trimmed the backing board to allow batteries to slide all the way to the back of the cabinet. To reinforce the cabinet, I had a piece of Steel bolted underneath the cabinet to the metal cabinet frames. It has worked well for me for about 6 years now.
2006 Seneca 35GS
2003 Subaru Forestor 5spd manual toad