A reciprocating saw would work best. The blades are short enough so that they "shouldn't" hit the outer door skin, but I would check this before powering up the saw. Another option if you don't have the power tools is to use an open-ended hacksaw blade holder. Hope that helps a little.
I used a Dremel tool with the cut-off blades. It may not be the most efficient method, but you can get VERY precise cuts. This was my main concern because I wasn't too thrilled with cutting holes in my doors to start with. I got excellent results with it. No drilling necessary.