Used the customizer on the original thing (link below), and since I didn't have a code on my key, I typed in 00000 to make a blank key. I downloaded it and edited it (removed the numbers and the letter on the back.
Once I had it printed, I took a pair of vice grips and clamped it to my original metal key. I then took a dremel with a cutoff wheel attachment and shaved the plastic key down to match the teeth. Took about 3 minutes to shave, and when I tested it on my apartment door, BAM FIRST TRY! Perfect fit.
Customized version of http://www.thingiverse.com/thing:52761
Created with Customizer! http://www.thingiverse.com/apps/customizer/run?thing_id=52761
Printed in Cura 2.5
Initial layer speed - 15
Print speed - 55