To hell with that!
There are many options out there that will help get around buying Nike's special shoes, but for those of us who run outdoors, and run long distances, we just don't feel safe mounting it above the shoe (at least I don't). So why not just model my current running shoes after Nike's special shoes?
NOTE: This Instructable involves cutting a hole in your shoes. I am not responsible for any damaged shoes, injured legs, cut fingers, or broken Nike+ kits. '
All of the instructions are detailing how I did this, how you should do this depends on your shoe, shoe size, and preferences, but it should be mostly the same (just different measurements involving sensor placement)
Step 1: Tools/Hardware Needed:
Foam Core/Mat Board X-Acto Blade (optional, but helpful)
Step 2: Measuring
My measurements are (you should measure your own to double check my measurement):
1 3/8" x 15/16" x 1/4"
Step 3: More Measuring
With my foot I had about 1- 1/2" in depth of area that my arch was raised. Obviously while running this will change, but this seems like one of the best spots for minimal pressure and stress.