Do you have an older-model refrigerator? Before they went all energy-efficient, the top of a fridge was nice and warm and GREAT for starting flats of seeds.
You can also use a person-heating, heating pad - just make sure you put a layer of plastic between the pad and the flats so no water gets onto the heating pad.
So, heating pad, layer of plastic (trash bag would be fine) then an old towel, then the plant flat.
[Cue Elvis voice ...] Why thank you. Thank you very much ;-)