Thanks CA Traveler - I don't have vertical access so I would need to load the batteries and then slide into place given the base of this box is 13" tall. It would also make life really difficult to connect/fill/maintain at the height of this box.
I was considering installing a sliding tray into the compartment but it would further reduce the height of my opening (currently @ 14.5") and probably not allow this box to work. Also would need serious anchor for the tray frame to handle the 120# cantilevered force when opened out.
Still believe I am back to my 2 options listed above.
A wet cell GC2 just does not seem in the cards without a custom built box - which may be doable. Fiberglass lined 1/4" plywood with .75 x.75 framing?