I use heating cables buried IN the seedling flats (with soil-less potting medium). Once the seeds have germinated in the flats, the cables aren't needed any more. Transplanting is the next step.

My heating cables are secured to the bottom of aluminum shelving and plastic seeding containers or cutting starting containers are placed right on the aluminum. Al

I used heat cables for years before buying a heat mat with thermostat.
I had a wooden box made for me for the size I wanted. Went to the hardware store and had them cut metal hardware cloth to a bit smaller size of the box. I use plastic cable ties to tie down the cable to the hardware cloth. Placed the cable and cloth on the bottom of the box and added soiless mix on top.
This arrangement worked great, until....... A new kitty decided the seedling box was a great litter box! I lost alot of seedlings, gave the cable to the local convent nuns and bought the mat/thermostat.