I put an extra pair of 6V batteries in my front storage compartment. I couldn't find any weight ratings for the compartments, but I built a wood box to distribute the load across the floor and to enclose the batteries. I'm not sure about a 2003 but my compartment is very stout. MY box is vent on the bottom and ontop in the back to the outside. Also I put a screened vent in the compartment.
I think welding new battery racks like NWjeeper did is a better idea, but I was in a hurry, didn't have access to a welder, and didn't want to pay to have it done. If I add another set I'll probably add them behind the grill like how NWjeeper did it.