I have posted about this several times, but it seems relelant again so:
I installed two golf cart (6 volt) batteries in my 2955 RL. I used a BlueSea battery box in which those two batteries exactly fit:
I was able to use the existing vent as well as the existing hole in the bottom of the compartment. Here are some notes (DOC file):
http://www.dw4u.net/box.doc