Turns out I could fit the thumb onto my cut off saw:
It was a chore to cut and the thumb and/or saw would move as I was cutting. When I was done, I was shocked to discover I had cut off 20mm instead of 10mm. I did a little bit more than 10mm on purpose as I wanted to make sure it would slip on. I guess there was just a lot of wobble in the abrasive disk.
Anyway, as it turns out, the piece I cut off made for a nice washer to make up for the gap:
I suppose I could weld it back on one day, but having a thick washer should work just as well I would think.