Foil and a few bricks

[bencops]

The clay tray is a heat deflector, it can't stop heat spikes in the system but can diffuse the spike over time and across the area of the cooker. Water can remove heat spikes and keep temperatures down as it evaporates from the cooker and takes its energy with it. This costs more charcoal.

However it turns out (from experiments) none of that matters to the food. I am a convert to the clay tray.

I use a clay tray rather than sand just because its easier to handle, but they do exactly the same thing, just a barrier to the raw flame and heat diffuser. Remember once they are up to temperature (under an hour I would have thought) they do nothing to remove heat from the system.