I had to tighten the new castle nut on my rifle and had lost the proprietary tool for this task. I was in a hurry so i printed one! Designed to be slid over the buffer tube and tightened with a pipe wrench or similar (clearance: 39 mm).

I printed several of these because i thought they would break easily made out of PLA, but i ended up needing just one to get the job done! I believe this to be stronger than the "conventional" designs also found here in Thingiverse, but haven't tested these different models against each other.

There is also a "filler piece" to insert inside the slot to supposedly make the tool stronger. The weakest parts are the small tabs that lock inside the castle nut, so i believe they'll break long before the body of the tool. Haven't even tested the filler piece, so it might not fit properly.