how to cheaply heat a cold frame

I have a small cold frame that I want to heat, to start tomato plants in. I am not adverse to using electricity, but heating cables are way too expensive to buy. I used a heat lamp last year, but the bulb exploded. I can't use manure, since the scale is just two small and I don't have a good source.

Any ideas?

Hot water.

Put a 2-gallon bucket of hot water in the cold frame. If it gets too cold at night (like <70F), then put a small aquarium heater in the bucket of water. Most aquarium heaters have a dial setting so you can set them between 70 and 85F. If you keep the bucket of water in the cold frame at 80F, then you should trick the tomatoes into thinking it is >70 and they will germinate properly.

I have heard of people using these rope light things. Like a bunch of small light bulbs in a plastic tube... IDK, we have a walk-in coldframe and grow lettuce, spinach, and radishes pretty much all winter, no supplemental heat, although I can sure see where the stuff would grow a wholelot better if there was a little bottom heat, esp in the single digit nights....