Does Shires still sell a bell mounting kit? I reached out to them but no one responded.

Or is there an “after market” option?

They still sell them. They’re 125$

They sell it, the threads are different than what instrument innovations sells. I bet you could ask the Olsons to make you some bits with shires threads. The Shires kit won’t fit your bell perfectly until after some annealing and burnishing. To save some money, I bought the threads from Shires and soldered it onto the existing diamond flange I had off my Holton bell. I had to remove material off the threaded post to fit inside the socket.

Anyways, Shires sells them, only takes a coul weeks of harassing but once you get in touch it’s a very fast process. They are nice people, but have too much on their plate maybe, I’m not sure. My friend is going on 9 months for waiting on his custom bass.