All illustrations by Alan Batley
Take a clean margarine or ice cream tub with a lid. Cut out the entire bottom panel, then the central section of the lid so just a frame remains.
Cover the sides of the tub with gaffer tape, or paint them with waterproof black paint. This will prevent the light from getting in and reflecting off the water.
Stretch cling film over the top of the tub, then replace the lid to hold it in position. You have now created a ‘window’. If the cling film tears, simply replace it.