One of the ways to attach a sling plate to your shotgun is to get a sling plate which is installed between extentsion and magazine tube. There are many sling plates available, here is one from S&J Hardware:
S&J Hardware Sling Plates

One of the ways to attach a sling plate to your shotgun is to get a sling plate which is installed between extentsion and magazine tube. There are many sling plates available, here is one from S&J Hardware: