I used a heat cable for years zip tied to hardwire cloth, wooden box with thick styro on the bottom, layer of sand, then the cable/cloth and soiless mix on top of that. Most seeds were up in 2-3 days in the unheated basement. All was fine till last season my new kitties decided this prop box was a great litter box. So now I have switched over to a heavy duty heat mat with wire cage in a wood box for 2 flat starts. I haven't bought the thermostat yet until I do temp trials. So far the lights with mylar and plastic over are keeping the soil temps at 72 but dip to 58 at night, so I will put a timer on the heat mat for when the lights are not running and test those temps. I had the lisianthus start to pop up in 6 days using this box.

Heat mats were the greatest improvement I made in my seed germination system, last year. I had three 20" x 48" mats, all running off the same thermostat. They kept the germination trays at 72 deg. F. I use the rubber encapsulated 107 watt units sold by Farmtek (and others) for about $75 each. I also use a remote temperature sensor with a probe on the end of a cable. I plug that probe and the probe from the heat mat thermostat into the same cell in one of the covered trays. Here is a picture of my setup. I am adding another mat this year. This setup has a large production capacity, with 4 mats you can have 16 trays going at once. Of course, each tray only spends about a week on the mats, until germination occurs, then the tray gets uncovered and moved to another light bench. I use a sheet of 1" foil covered urethane foam insulation under the mats. This improves the efficiency and reduces power consumption. Here is a link that might be useful: Farmtek Germination Heat Mats

Look on Ebay. You should be able to find a single flat size for around $20. Just check the feedback rating. I wouldn't bother with a thermostat. Karyn

OK, this isn't the knowledgeable input you're looking for (g) but...our breaker boxes (we added a second one during this remodel) are at one end of the house and the master bath is on the opposite end. We added two fixtures (air tub and Washlet) each requiring their own GFI. Heater/fan...I used to wonder about that too. Finally got up the courage to ask (lol) You simply don't use both at the same time. I have the Panasonic Whisper Warm and am loving it this winter but don't use both features at the same time. I use the heat when I'm not in the tub but if I've got the air tub going I'll use the vent for the steam issue. As for the heating mat in a shower. Some here have done it. Contact the manufacturer and see what they have to say about it. Monica