I'd been thinking about how to add more shade for the hive when I noticed a large piece of Masonite in a hard rubbish pile nearby. Needless to say I grabbed it and it is now being used as shade for the side of the hive. Leaning the Masonite against the hive stops direct heat hitting the hive which has a beneficial effect. Plus it's heavy enough that it won't blow over easily - not too much of issue for us as our hive is quite sheltered from wind.
With the combination of the broccoli box full of water and foam floats on top of the hive, the cardboard shading the top box and the Masonite shading the side, the hive looks like a bit of a mishmash ...but if it helps keep it cool then that's all that matters.