Sorry I missed this earlier. Are you in the Uk? Heat mats like those for reptiles aren't really safe, also, direct heat shouldn't touch an enclosure. There are other options. We have an oil filled electric radiator with a thermostat and a timer in the room where the hamster is. It comes on for maybe 2 hours in the middle of the night (or sometimes twice a night), but only comes on if the temperature drops below what you've set the thermostat at. It also has a safety cut out. People also insulate cages/enclosures by wrapping things round the base and you can use Snugglesafe microwave pads near, but not touching the side of the enclosure.
If you have central heating though, having it come on for a couple of times for an hour during the night would be the best way - although then it heats the whole house rather than just one room.