Aero Chamber Plug (aka asthma inhaler spacer rubber bit)

Replacement bung/plug/rubber thing for AeroChamber asthma inhaler spacer.

Rotate part to be flat, print in TPU/your best slightly ridged flex filament, cut anything stuck together that shouldn't be, slide on and suck your breathing problems away...

50% infill seems to work about right as the outer lip is a bit flimsy, but I'm not your mother, do what you need to make it work, and tidy your room too.