This is a Pmag +15 extension I modeled to further my skillset with CAD. 
Fits Pmag Gen3 and accepts oem baseplate and locking plate. Parts are secured with one M3x5 socket head bolt.

You'll also need to file the nose of the mag follower as shown, this can be done with a small round file. This prevents the follower from catching the lower edge of the forward spine inside the Pmag body when the follower travels up and down past the part line.