Afternoon Brian,
I thought of the drill and tap method also. The flat steel I used was 1/8" (easy to cut, grind, etc.). Just to be on the safe side, I would also place a nut to lock the threaded bolt after I screwed it in where I wanted it. So, I would have a lock nut on each side of the threaded rod.
Guess I'm just a little overboard, but shooting the .375H&H, I was just wanting everything to stay secure with all the recoil. This way, nothing moves. Let me know how it turns out.
Karl