Cover for a hot bed

[Aberdeenplotter]

My bench would hold 20 trays but don't envisage having that many at the one time, so will be using part of it to grow on some pot plants, I have never thought of the second poly layer but like the idea as it would help maintain the temperature, and hold the moisture around 5he cable (I have had a bench dry out and go on fire, it was peat that I had been using rather than sand) do you have the capillary matting drawing from a reservoir or do you wet it down with a watering can

I just water it ever so often. I can see how peat wouldn't be a great idea