I basically cut the hole deeper with a kitchen carving knife. Rough, but you just need the hole to go all the way through the styrofoam.

I ran the cable along the passenger side following the factory wiring routes. I had a 25 foot cable and only had a few feet left over just before the changer, so make sure you get at least 25 feet.

Two hacksaw blades cut to the proper shape would indeed work for the 'keys'. Here is the pattern I used for mine.