I used PLA. It’s not the best choice but it’s the easiest to work with for testing and iteration. I have about 3kg of ASA filament sitting in my stash though and when I’m finished making an enclosure for the printer, which is the current project I’m working on, I’ll see how well it prints.
ASA has the main benefit of UV resistance, increased strength, and higher melting temps but I expect PLA should work fine for this part though as it’s not supposed to stay attached all the time on the boat. I remember reading that the boston valve isn’t totally sealed unless the cap is screwed in. A solution to this might be to glue the tip from a top-up valve to the vinyl tube.
As for the threads, they are weird. Definitely 14TPI, so the thread pitch is the same, but almost 2mm wider than standard 7/8.