If you have normal hive, 15W reptile heater cable is the best and cheapest.
You may get lower wats with electricity timer. 15 min on, and 15 min off. 7 W is as good to one box give.
In double brood 15 W is good.
At least you will see, how important heat is to colonies.
You can use such cable in extraction. When I uncap combs, I have a heater cable under the mesh, where cappings drop. It keeps capping uncapping honey warm, and it drills from wax better.
When I keep queen cages on the cable, queen and assistant workers do better in that heat than in 20C room temp.
.