Thanks for the reply, one way valve seems like a good plan. Surprising mira would recommend something that breaches the seal. Saw another article where someone had drilled a load of holes in the actual tubes themselves, they seem to find that did the trick but can't see how it would help with...
Just ordered this tray and thinking I'm likely to have the same issue. Does drilling the hole on the waste tube (presuming underside) mean I'll get sewer smells as breaching seal on the waste? Many thanks for this btw