Marcus Metcalfe, of DIYFLOATROOMS.com, has done the open source tank community a huge favor by posting photos of the DIY floatrooms he has masterminded:

I phoned marcus to learn about his heating and insulation system but got no answer. With Google Lens, I probably could’ve figured this out in a jiffy. At any rate, at Marcus’ facebook page we see these key photos:

Now, about 2 hours later, I found floor heating mats at walmart and then an equivalent number of such products at amazon.

I’m concerned about the low wattage in these mats (200W)… but because they are spread over the entire surface of the tank and because they provide uniform heat to the tank itself and not the water, I suppose Marcus found that workable?

I’m currently using 1000W to heat the water directly in my DIY tank… but also: 300W waterbed heaters have been sufficient for many people as well. In fact, Samadhi tank company apparently was using such off-the-shelf heaters as late as 2013! And another beautiful build apparently uses the exact same waterbed heater model.

So it looks like such high wattage is only necessary for directly heating water.