This is a thumb hook that replaces what usually is a mildly uncomfortable ring on the tuba. It has an integral support and skirt to prevent separation from the printbed while printing. It only fits on tubas that have a screw-on thumb ring, and requires a screw or bolt (M6 thread typically) in order to be mounted.
Note: The reason there is typically a ring is so you can lift the tuba from that point. With a hook, you can't do this. You also cannot invert this hook, because it won't support the weight of the tuba.

Doesn't Matter

No

100% or close to it (for strength)

This piece has to be strong. It will take a while to print. Do not waste your time trying to make it lighter.