The system has worked very well but I didn’t like the way it looked.
The materials are very simple; some plastic foam (I used a slice of a large pool noodle), a section from a plastic food container and the ever-reliable duct seal. Note that you must use duct seal and not plumber’s putty.
For an even nicer look I will probably replace the ring from the yoghurt container with something like a slice of PVC pipe. The ring is the perfect moat to hold the 2 tablespoons of water needed to “see” through the hull.
The pictures tell the story.