I have a 15" MacBook. I've also had at least three chargers, because all of them keep fraying at the base of the plug.
This time I just decided to fix it myself.
Use a pair of needlenose pliers. Flip up the cord storage clips and put the jaws of the pliers in there. Now, pull the pliers apart. They work like a miniature "Jaws of Life" and put prying pressure on both sides of the plastic. With a little finesse, the adapter will split right down the seam.