I'd already made the box when I remembered Kiteman's Zeroth Law so the photos don't show the whole process. Remember to set the focus first if you're using a manual focus webcam; I used about 8" (200mm). The webcam is held in place with 4 dabs of hot glue. The PSU block is only for ballast, but I needed the tiles too to stop the box floating. Seal the cable entry and the box join with hot glue.
This took about ten minutes to make and I'm treating it as a proof of concept for a more engineered version on a pole to use later in the year when the pond plants have grown and there should be some newts around.
Warning - Bubbles coming from the box are a bad sign. I had to get the webcam out after 20 minutes or so due to a small leak - those boxes are very brittle!