If it helps I use a ferry-morse 10x20 17.5 watt matt beneath my propagator. What I found was that in my hands a temperature controller was a must (its a sad story lol). I happened to have an older Johnson controls A419 kicking around which I set for 74F. If your not a control electrician I'd recommend some form of temperature controlled mat however you care to achieve this. This works wonders for my paper towel method and then after with my doubled keg cups still in propagator.