Thermodynamics: Heat will move from a warm spot to a cold spot more readily than from a warm spot to a hot spot. Put something, like a very cheap polystyrene insulation board (cut to size) under your heat mat, replace your pots, then try strips of newspaper or similar over and between the cuttings to act like a blanket, keeping their 'heads' exposed, of course. The cold spot will be your cuttings.

I agree. At the most basic level a piece of cardboard wrapped in foil under the heat mat will help. I have used an old accordion style dashboard sunshade made out of aluminum foil and thin foam, old election signs, garage door insulation panels, and flat sheets of foam packing materials.

I have also used "Window Sill" style heat mats (long and narrow) and wrapped containers individually or in small groups with them. Thus giving much more surface area in contact with the heat.