I don't use CHEs but I think 60 watts may be too low for a 20-gallon tank - I believe 100 watt is what you'll need to maintain ambient temperatures. Hopefully someone who uses CHEs can chime in here.

Just a quick note to add that the towel on top for retaining humidity is a potential fire hazard. Instead, you can tape the lid other than the area under the CHE with aluminum tape/HVAC tape.