I have done lots of these...I use Insul-Brite by Pellon (I think) and two layers of Warm and Natural with no problems...I even construct hand pockets in them ITH and they also have a layer of batting with fabric and lining...You might need to use a size 80/12 or 90/14 needle, depending upon how thick you hot pad gets...I have not had to change the tension on my Baby Lock Ellisimo abd gave not broken a thread or needle...Here is a link to ones that I did awhile ago...
I used the stuff that's thin with silver on one side; I think it was Floriai product, expensive but a yard makes many Then I added a layer of fleece, too. I don't want my hand burned! It stitched fine. Have not applied my binding yet - doing a stack of about 30 patchworks for "the booth" and for gift giving.