Mcmaster or fastenal carries the plunger. We had them on hand at work it's a 5mmx15mm was slightly too long so I just lock tighted it in and smoothed off the back. The plunger works against the o ring to pull it together nice and tight.
Interested in anybody fabbing up farkles to make the sport more enjoyable. I just turned out these soil cleats for my atlas bipod. They are stainless but I'm thinking the next set should be aluminum.