All of the heat mats I have used will heat in both directions - that is to say they will lose heat on the bottom side if not set on an insulated surface. There may be other sorts of heat mats that I don't know about.

My favorite insulation for such things looks like bubble wrap made from aluminum foil. It is easily cut to almost any size or shape and if one ruins it there is no considerable loss.

The info that came with my heating mat said not to put it on metal surfaces because of heat loss below, It said the best surface is styrofoam, cardboard or wood. I use a cardboard box lid flap to put mine on and it has worked well for 4 years now.

I suggest you take a cardboard box and cut it up to fit your shelves where you are going to put the mat. Put your plants that are in pots in a plastic tray and onto the mat and you are in business

i put mine on foil backed insulation. it comes in several size sheets and different thickness. you could cut a 4'x8' sheet into whatever size you needed.