I use a Medium Size Harrison Dark Cloth - http://www.badgergraphic.com/store/c...t_detail&p=670

The silver outer face reflects heat from the sun but even so gets quite warm in there after a while

Its also waterproof - so use it as a rain cover for the Camera when its set up on the Tripod

There is no right size of Dark Cloth - they need to be big to shut out enough light but small so they don't act as a sail in the wind (I have learned to keep one hand on my Tripod at all times when the dark Cloth is on the Camera)

Have fun