Before using a heating pad, which could alter a portion of the mash temperature, but not likely maintain the whole, I would try the foil bobble wrap and a sleeping bag or a thick wool blanket. I use both in the middle of winter with much success in my garage (to keep the wind off it). A 90 minute mash will see measurable loss, but typically a 60 minute mash will reflect almost no loss. I well insulate the top and then a little less insulation on the sides.
Best of luck - it can be done!