Tom,

I doubt it is a hazard but you may want to see how much heat you lose if the
mat sit on an open shelf. My old heat mat (that I don't use much right now)
is encased in a metal frame that suspends the plant tray about 3/4 inch
above the matt.

You probably want to place the container that will hold your small plants on
the mat and put a thermometer inside to see what the temp is at various
times of the day.

The old mat I have will get quite warm.

Let us know how it works for you.

Good Growing,
Gene