The way I see it, you have four options.
• Pay the price for a genuine Willson part that has to be custom-made for you. (That is actually a decent price for a custom-made part.)
• Have a ring made for you by your tech, which will also be costly. And it will take a long time for him to source a suitable ring and mounting system.
• Leave it as is, sans ring.
• Post an ad in the WTB Forum asking for owners who have one they would be willing to sell to you.
On my Kurath F (his direct predecessor to the Willson 3200) I installed a ring from Jürgen Voigt in Germany. These are the same rings used on Meinl-Weston piston tubas. Your tech would have to set up an account with them, order and pay for it, and then wait up to a month or so for it to arrive.
He will have to email them as they are not set up to deliver products to the US through the website. But they will happily set this up via email. (Yes, it is a hassle.)
This currently costs a bit over $40, with shipping from Germany making it probably closer to $75 or $80.
https://shop.voigt-brass.de/en/B2B/THUM ... table.html
Here is the valve section of my Kurath with this thumb ring installed. (The earlier Kurath had a more primitive bracket for the thumb lever, and the ring was separate. It was not adjustable like yours is. It was in a terrible place for my hand, so I removed it. Later I decided I wanted a ring, but it needed to be adjustable enough for me to dial in the location just so, I had planned on replacing it with a fixed ring once I had determined where the thing needed to live. However, I have used this Voigt ring happily ever since. It is quite strong and well-made to be sure.
The alternative to this is having your guy source a suitable ring. Most stuff like this in the US is going to be something akin to a Conn or King sousaphone thumb ring, which will be too thin, too small, and quite ugly in my book. They are also easily bent out of round. A really heavy ring like I linked to above is much harder to source and might be costly. Then you have to figure out how to mount it with an embedded threaded post that is strong enough.
Good thumb rings are not exactly cheap. The ones on the Kurath/Willson tubas are HUGE like old Cerveny rings. Here is mine, after I deleted the terrible backing plate. I hope to reuse this on my Kurath in a few weeks. I have to make a mounting plate similar to yours as the one that it was brazed to was junk.
Please note that the Voigt ring is made from a solid nickel silver rod and is heavy. The Kurath, like the fat-boy Cerveny rings, is made from nickel silver tubing. It is just as rigid, actually MORE rigid if you understand structural integrity with solid bars versus tubes) and is very light and strong. Just giving you more info.
The best option is to post an ad searching for one that an owner no longer wants.
Good luck!