Firewall for Trunk w/ Nitrous and battery!

They have pre-made boxes for batteries, although it's kind of pricey http://www.rpmoutlet.com/musv6batt.htm
And you'd need a pretty stinkin' big switch if your going to put it in line with the main battery cable if that's what you mean. But you could run wires from stuff like the fuel pump, ignition coil, and the nitrous systems power cable (although I'm not sure how the nitrous connects), through one single cutoff switch, that should set you up. And before you do anything, I'd look over the regulations at the local tracks to know what you need. It would be a bummer to set something up and it not pass inspection.