One of the best, if not the best, bases for high capacity magazines is made by Bevin Gram with Grams Engineering in Costa Mesa, CA.
http://www.gramsengineering.com/
I have all my P-14 mags with his extended base pads and they now carry 18 rounds. I also have my Glocks with Bevin's pads (G-21, G-19), as well as my competition gun in 40 S&W. That carries 19 rounds. The pads are easy to clean which is a big plus when shooting matches, and he is a magician in twinking mag tubes. He also has other items for the shooter worth looking at.
Good luck and have a Happy New Year!