I am not heating the enclosures directly, they are too small and I will only end up cooking the poor slings if I try to heat them directly.
I use a thermostat controlled heater in the room which keeps room temperature between 15 and 20 degrees on average at the moment.
The slings enclosures are about 10cm x 10cm x 10cm and there is enough humidity to condense a bit on the inside of their enclosures. I keep the slings very well hydrated without soaking the substrate, there is usually a drop or two of water forming on the roof of their enclosures.
I have also noticed that they aren't eating anymore so I am hoping they are in pre-moult. I guess I will know within the next week or so, but I am feeling slightly left-behind since it seems everyone else's slings from Cerat's brood have moulted once or twice