It looks like the sort of thing that small precision parts sometimes come in. Or something like a tap or reamer bit.
I wish I could figure a way to make a decent spit catcher for my old Buescher tuba. The problem there is that the bottom caps don't have the sort of grooved nipples that are on more recent instruments. I suppose I could bore out the bottom holes, tap them, and screw in something appropriate. Maybe some day ...
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)