My batteries did not fit my sort of recessed tray as well. I just ran a couple of 1" by 1" by 36" angles across the top of the frame and now I can mount up to four standard plastic battery boxes side by side. The stick out a few inches on both sides of the tongue now.
My three batteries are inside one of these standard inexpensive WALMART plastic cases..

When you get back from trip then you can arrange to get a metal battery box across the top of the tongue area bolted into place.
My next UPGRADE for batteries will have an enclosed metal box like everyone is talking about (Including myself) for better thief protection...
Roy Ken