I made the one for my euphonium from stock PVC plumbing tubing and a couple of plastic end plugs intended as hole plugs for wiring holes in desks. All purchased at that mecca of brass instrument supplies: Lowe's. Then I added an Amati water valve to it so I don't have to take it off to drain it. It fits firmly, doesn't rattle, works well, and didn't require any "machining" beyond an electric drill and a file. I was going to make another for my big Eb tuba, but discovered that with a bit of judicious plastic shaving, the one I made fits both the euph and the tuba equally well.
Given the cost of these things from instrument supply sources, someone could probably make some real money by making and selling plastic printed ones. I've seen that done in some other "parts areas" on Ebay for rather hard to get but easy to print small parts (such as single-shot magazines for target rifles). But I'm retired and not looking to start a home industry.
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)