Since heating pads and electric blankets come with built in rheostats, you would have been better off with a foot pad with such a control built in.
Actually, a good dog bed warmer would have worked and they have controls. I think trying the plug in rig won't harm anything.
The towel will just get as hot as the pad, how about a piece of lattice or some sort of gridwork that would allow some air to circulate and reduce trapped heat?
The bottom of a plastic "flat" that plants come in from the nursery comes to mind as probably laying in the backyard. "REPURPOSE"