I don't know if this will help but we added two "Interstate" 6-V Batts (GC2-XHD-UTL) to our Flagstaff V-Lite 26WRB. Our dealer supplied the heavy duty opaque plastic box which has inside dimensions of 21" X 7,1/8" X 11,1/2"deep. The batteries are 10,1/4" (2) X 7" X 11,14"high O/A. They are a very nice tight fit but require a battery carrying strap to remove. I am attempting to attach photos but have not got the hang of exactly how to do this from a Mac but will give it a try.
If they attached they are mostly self explanatory but the open box does show just one 12 Volt batt. This is just an old one I use just for storage. The two 6 Volt units are showing on floating charge in storage.
I modified the lid closing arrangement as the dealer had just used plated sheet metal screws. It can be seen that I used 1/4" stainless threaded inserts. On the lid I use four SS 1/4" bolts with flat and lock washers.
Hope this is of some help.