I found your terrific idea while plodding through the forum tonight This seemed to be just what I needed...a good way to store 6 volt batteries. I decided, though, that this is a lot of extra work (I don't have much extra time is the problem) so I will instead will cut about 0.75 inch from the coach-side lip of the box I have, and about 1.25 inch on the propane bottle side lip. That will give enough clearance to set two 6 volt (7 1/8 inch wide) batteries side-by-side in the metal storage box.
Then the two existing, but too short, storage bins with locking doors become storage for things like tools instead of places for the batteries.
Then I just need to design and build a matching cover for this battery box.
This is still an excellent idea Wasafari