A friend and I published a study that we did about protecting the bonsai root mass from solar heating in the ABS Journal a couple years back (American Bonsai Journal Volume 53, number 3), . It can be accessed here:
The bottom line? Of course the first choice is moving to shade and repeated watering, but if you are unavoidably away from your trees during the daylight hours, the most effective strategy that we found was covering the entire pot and soil surface with white cotton t-shirt material. Covering with other non-permeable or insulating materials appears to impede radiative, evaporative, and/or convective cooling, which is crucial for the pot, soil, and root mass to give up any absorbed heat. The cotton reflects a good deal of sunlight but allows the pot to breathe.