cr0sh:
What do you need the 12 volts for - the router? Whatever, it would probably be easier and better to get a couple of 7.2 volt packs, wired in series (make a "y-cable" that allows for this, so you can disconnect each to charge the batteries), then drop the voltage down with a 7812 regulator (and if you need the current, use a bypass transistor - or even better, see about getting a drop-in 3-pin switching regulator for 12 volts). Ultimately, you're likely to get a better system with a longer runtime going that route. So far, though, your machine is looking great!
I found out the router will run on less than 12V, since it regulates the input voltage down to 3.3V anyway. So, I'm feeding it with the 10-ish V battery.
We decided to build in the router, since it fits snug in the roof compartment. We extended the antennas with TNC cables, and made them pop out of the back window. They're nice and secure, and still turnable, and easily removable. If anything, we're getting a better signal.