To attach the heating element to the transducer, I ended up 3D printing an adapter. It turns out it's extremely hard to find M10-1.0 bolts. I only found some that were used on some cars. So instead I modeled a little adapter block in Fusion 360 and had it printed by Fictiv. I couldn't print it myself because this part has threads that need to be accurate.

One side has the M10-1.0 thread and the other side has 3 screw holes to attach the heating element. I probably should have made the holes for the screws slightly larger. I also should have used threaded inserts instead of screwing directly into the plastic. The plastic held up ok, but it started to crack and lift in some areas and I'm hoping it will survive the 40W of ultrasonic energy it will experience. If I still had access to a machine shop, I probably would have made this piece out of aluminum. That way I wouldn't have to worry about heat leaking from the heating element mounting holes into the adapter, and I wouldn't have to worry about it failing during use. This piece would be pretty simple to make with a lathe, drill press, and threading die.

I thought about it more later and realized that I could actually make this part without support material by using a threading die like this: https://ultimaker.com/en/community/9671-tap-die-testing

Here's the whole assembly so far: