As noted, the PAM-2 is one possibility.
The other is the Madsen M-50 magazine. For those, you need to cut a catch in the rear rib as well as to press flat or grind out the Madsen's catch on the front of the mag. They fit a little sloppy in the M11/9 magwell, but still run fine. C&S Metal Werkes converted a bunch of them, back in the day.