They were set up lean to fashion with a hole underneath to record
temperature. There needed to be at least 3 feet from the thermometer or the heat absorbed by the plastic would radiate to
it. It was so hot that we had to wear boots with thick soles and
wool socks or your feet would burn from the ground.

The coolest places were natural shade spots, as they handn't been
heated up prior, and were the most opaque of all.

Yes, clear plastic makes a better solar still, but black works too.
A friend of mine traveling in the Outback of Australia would tie
black plastic garbage bags around vegetation to collect the water
that would cook out.