I also trimmed a little of one of the plastic lids to get 2 boxes to fit.
Last year I bought an aluminum box to hold both batteries. It was just big enough to hold 2 gr24's and still fit on the tongue. I went for the black powder coat - looks great. A little pricey, but with extra wires for my solar charge controller and inverter, it cleaned up the wiring to have just one battery box. Only downside to this box I that it isn't quite tall enough for 2 6 volt batteries, but perfect for 2 12 volt batteries. The 27" wide version is taller, but was too wide to fit on the tongue without moving stuff around.