Awhile back, I modified a Blackhawk pocket holster for a P32 to carry an extra mag. I’ve since sold the gun, but I found the attached image showing a similar modification done for an LCP. If the mag in the image was rotated 180 degrees, it wou be a more streamlined fit. Not the prettiest solution, but it functions.
You need rudimentary sewing skills to slit one seam and then add some reinforcement stitches to keep that seam from opening up along the edge.