I used Loc-tite super glue gel to attach L-clip to mag. Then I shim the mag in place with wide popsickle sticks (not the "skinnier style sticks") on all sides (front, back, right & left). It takes a few times to get it lined up to feed cartridges properly, but it works. You may have to put popsickle sticks in the bottom of the mag well on the stock to raise the mag to the correct height.

FYI - i did this on B&C Duramaxx stock.