Thanks for all the suggestions. I tried them all but had no positive response. Caravancrazy responded with a request for photos which I tried to send off but have had no further communication since.
I then remembered an old friend who lived locally and was a trained engineer. He came round to have a look and after a long discussion suggested we try purchasing a 640 ml diameter plastic sheet to fit over the existing perspex which would rise and fall with the venting of the washroom but seal up the aperture when travelling etc
I could not directly order some plastic, there is a world shortage but one company are receiving an order next week and have asked me to ring in a request 2 days before delivery. Here’s hoping!