Ever since I read about people using oddball heaters to make DIY heat mats, I've been keeping my eyes open for some large box I could insulate, to heat 4+ trays. It would have an air space to diffuse the heat ...
But I don't really need it. And after I made it, I would have to find a place to store it 8-10 months of the year.
I do have one commercial heat mat that has some thick rubberized coating. It looks identical to every heat mat I've seen advertised.
Even so, I lay plastic film over it to help keep water away.
