We have a problem with a drain smell in our cloakroom, which is fitted with a shower. I realise that it is possible to fit a non-return valve to the outflow, but access would be very difficult. Then I came across an Australian non-return valve which might fit in the shower drain:

https://www.youtube.com/watch?v=P1bbwCyHyn4

And I also found "floor drain duckbill check valves" - but no associated web page.

Has anyone come across these, or anything similar?