I am in the same boat looking for a solution for my Eberlestock F1 Mainframe & Transformer combo. From some reading on here the consensus seems to be three options: Homemade, Stone Glacier Sling, or Kifaru Gun Bearer. I am leaning towards the Universal version from Kifaru Looks like I could attach the "bucket" to the frame, and secure the forearm/barrel with either the compression straps on my pack, or use the under arm option situation depending. Let me know if you find any other options, I will report back once I get mine rigged up.