If it helps, I made one with a heating pad set on low. At first I thought it would cook the plants but I placed a thermometer under the humidity dome and it regulated quite well.

I place the mat under a tray of water with a few risers (1/2" x 1/2" x 8" blocks) and the tray of seedlings sitting in/near the water and placed the clear dome over that.

I never has such success with seedlings before.

If you are more industrious, you could use a variac to regulate the heat but most people don't even know what a variac is let alone having one. (-:

You can build your own with a double gang wall switch box, a dimmer, and a switch/outlet receptacle.

None of these ideas have a thermostat but as I said, my heating pad regulated quite well

Thanks so much for the tip. I will try your suggestion.

Also...old waterbed heating mats work great, and they do have a thermostat...ask your friends and family..they might have one kicking around!

A DIY root warmer can be made with a frame made from 1" X 4" boards cut to the desired size to make a rectangle the size of an old metal cookie sheet. Screw the frame together; mount a light socket in each end for a 15 watt bulb; screw on the old cookie sheet. Set plants on top. If it is too hot, place a pencil under the plants to allow air circulation.