What a fun herping trip! I Love Turq, but they both look great! I wouldn't worry too much about heating at this point. 67 overnight is fine. When you can afford it, the thermostat that Steve recommended is awesome. It just takes all of the guess work and tinkering out of the equation. The small heat mat is likely fine for the 20L as well. I don't use extra heat for my garters and they seem to thrive. That said they absolutely love a basking lamp, so you're off to a good start there Just remember, especially as adults, these guys really are pretty hardy. Too cool is better than too warm, within reason of course