Under tank heater for 20 gal, 24 wide x 17 tall x 12 deep ??

[Shade38]

I have a tank 20 gal, 24 wide x 17 tall x 12 deep to house red eyed tree frogs. I'm looking for an appropriate "Under Tank Heater."

The substrate will be moss or paper towels. I do NOT heat my house so it's 60-70 during the day, 55-65 at night this time of year. Half the top is a screen, the other half is an 18" fluorescent light fixture.

So I'm thinking of Zoo Med Repti-Therm under tank heater, the large is 18 x 7 I think. Then there's another one that just says Repti-Therm (not Zoo Med so what the...?) listed as 30-40 gal, medium size. I'm confused. Thinking I may need a heater listed for 30-40 gallons not 20, as my house is pretty darn chilly.

Any suggestions for proper size/type/whatever would be great, thanks!! Also your own experiences with these. Thanks so much...

Toni