Well you were here and saw my heating pads on the plastic. It is safe. There is some thread about a rubber or plastic heating pad on the forum that can burn, but it is a pad that get very hot to start off with and the tinfoil kind no one ever reported issues about.

Since I know you will likely keep it in the garage a panel heater might not work and don't put combustible liquids next to the tub and against the heating pad. :dry: There might be fumes that can harm the roaches and/or a smoker walking into the garage - kaboom

Other than that, I see no problems using the tinfoil/silver heating pads.