I am going to try this, unless there is a reason not too.

Get 1 large empty squash container from asda / tesco. The 5 litre ones
with a handles.

Using a wood / spade drill, drill a 25mm hole in the plastic lid. The
bung will airlock will be a snug fit in the screw cap.

There you go a free plastic demi john. Also square for better storage.