What tool?
 

What tool do you use to cut the back window wider so the dual cell can fit?

I'd use an x-acto knife personally. Easy, quick, and sharp. :D

dremel

i used a dremel cuz i suck with an x-acto knife:)

Its a little bit messy, but you could also heat up an old knife or something over a flame, and it will slice through the plastic very easily. Then, file the sides to make it even.

i saw at canadian tire a soldering gun tip that was flat with a blade for melting plastic (i should've bought one)

yeah, that sounds like a good investment

i always thought a canadian tire was a snow shoe.

next time im there ill try to remember to get one but on my way there i should pick up some tim hortons doughnuts LOL:)

heres what i'm talking about
http://www.tool-up.co.uk/Images/00/00/21/00002184.jpg
and
http://www.tool-up.co.uk/Images/00/00/21/00002191.jpg

'm using a scalpel so could I just heat it up with a soldering iron?

i would think so:)

you could use a candle, but it will char your blade, though.