I need some box turtle help please.

[sandy]

I have ten boxie eggs in my incubator, and at least seven are fertile.
My question is, how can I keep the hatchlings (once they are six months old) warmed through the night in Winter?
I don't want to use a viv, I would prefer to use an open topped enclosure.

I know I can use a heat mat, but what sort is best?
I have found a water proof matt, but not sure if they work or not.
Or would an ordinary heat mat work?
Are thermostatically controlled heat mats any good?
I hope Dan can help, or of course anyone else that has boxie knowledge.