Theyre a little bit flimsy, but they actually work.
In his video, Dave Hax starts by lighting the key with a lighter until it turns black.
Next he places a piece of tape on a can lid and cuts around the outline.

Yep, its that simple!
Who would have thought?!
Now that dreaded trip to your hardware store to have a key made can be postponed.
Watch how Dave Hax does this in his step by step tutorial and make yourself a spare key!