Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
20 gallons is a very small volume, and I think that it will vary, even partially buried. If there is any way you can double the volume that would help a lot.
Having some shade in the hot part of the day will help, too.
Do a test run- set out something cheap, perhaps a kiddie wading pool in the full sun, see what happens.
Put it in a morning sun/afternoon shade location, see what happens.
Put a thermometer in it, and record it perhaps every 4-6 hours through the day.