I ate my tropical fish and used my aquarium heater to provide the right temperature level ... No, of course not!

I kept it more simple. I took a 30x20x20 centimeter cardboard box, wrapped the filter box in a warm towel, added two more towels to fill the box and closed the cardboard box with tape. I'm pretty sure the filter box is in complete darkness. I am worried about the temperature. I put the cardboard box next to a central heating radiator but it would be a bit expensive to keep my house at high temperatures for 3 - 6 weeks. My house is about 20 degrees Celsius, at night sometimes 18..

What do you think? I have no way of knowing how warm the filter box and the cake are inside the box. How do you keep the cake warm? Any of you using the aquarium heater method?