add cells without cutting wires!
 

Okay, this is basic, but.....if your battery case has long + and - wires like mine from RS does, then you can just desolder the negative wire running from one battery pack to the the other on both ends and solder your + and - wires from the battery holder the way that BC shows in his diagram. Easy-peasy!

and it'll still reach long enough to mount it in the trunk?

no.
 

Mounted in the roof. Not enough room for trunk. I've heard that a single cell will fit behind the bumped, but a dual is too big.