I used a multitool to cut the original battery box hole larger. I purchased a battery box that fit the 2, 6V Trojan T105 batteries perfectly. The battery box sits 3" below the top part of the floor and rests on the bottom layer of sheet metal and is supported by the rear bumper framing, where the shore power cable bundles.
I cut the hole in the floor precisely so that the battery box was a "press fit". That box can't move unless the trailer gets upside down. Then the battery box would be the least of your worries.....
The picture that you posted is of my 2004 2720SL. I did the exact same install on my 2009 2720SL. I just trimmed it with some Oak molding to tighten it up a bit.