I had this idea

If you are using Grow Pro nursery pots I found that you can put one inside the other and there is an airspace between them. This airspace would prevent the inside pot from direct exposure to the sun.

The experiment would be to put 2 of the same sized pots next to each other with a thermometer in the soil in each of them in the same location. One pot would sit on the ground while the other pot would be inside another pot of the same size sitting on the ground.

Rich, I would improve the experiment by using the outer pot being a bit bigger than the inner pot, that way you are providing space...
BTW, you could fill the space with Styrofoam, plain dirt, sand or even saw dust.
what you think?