I have heat mats with controllers
They have a cheesy suction cup with a temp probe.
You need to put something on top.
But you need to make sure the sensor stays and does not roll off.
I have been using coco core small pots that fit in a tray with a cover till they show leaves then I spray them
I set it to 80 degrees usually. I've tried other temperatures, but really haven't kept track of what works best. 80 has been working for me with both seeds and clones, though since I got an Oxy Cloner I haven't needed a heating pad for cloning. The pump that goes in the reservoir keeps the water warm.
The plant is scrogged out in a 2x4. I have a dome under the canopy with 8 new clones. Unlike the first one, these are not wilting....yet, and we are going on 3 days. Since I planned on taking the summer off, I am sure they will survive!