Updated photos in OP.
I've completed the updated feed plug. Overall, I'm very happy with how it turned out. The cap has the reinforcement bolt installed and two #015 o-rings. Initial testing resulted in zero misfeeds. I'll want to see how long the o-rings stay intact, but o-rings are cheap. If they prove to be more problematic than they're worth, It shouldn't be too hard to add a more traditional peg.
Edit: After some testing in the back yard, I tweaked the feed cap just a bit by extending the face by 2 mm and deepening the recess for the washer and nut in the back by 2 mm. Now the gun feeds even when tilted near vertically and the end of the bolt is below the surface of the rear side. Visually the second and third versions are nearly identical, but this small change makes feeding nearly foolproof. I'll call this project done for now.
Last edited by JonM; 04-07-2019 at 10:55 PM.