I have done a couple of 48-inch side mount tool boxes for friends for more battery space for the smaller trailer like the X213. Sure helps keeping the batteries out of sight - out of mind...
TORK-LIFT has a nice medium size aluminum box with slide-on lid that looks neat...
Sometimes the lid on the boxes won't open completely giving a problem to remove the batteries when needed.
You might check out your Bicycle storage area where the 12-inch wide very large floor shelf is installed. There is not much inside this large area. Might be able to build-in some storage doors here..
I would also investigate the storage area around the front clothes ward. There is a bunch of unused space you can see back behind this area accessible from the side front storage door...
I was looking at this trailer really close at one time to become my OFF-ROAD Ham Radio Emergency Comms trailer. Was wanting to remove both bunk beds and turn this whole area into a small storage area just inside the back door and the part in the middle would become a L-Shaped DESK area with hutch for my Ham Radios and computer setups...
Something like this
Would cut one of these OFFICE DEPOT desk kits down to fit...
Obviously I am not worrying too much about tongue weight haha...
Roy Ken