When I have had a hot pad laid out to sew on and I have to leave it I lay the pieces in order on a sheet of paper and cover with another sheet of paper. then staple along the sides leaving one side open if you wish or staple all the way around. Now it will stay organized until you can get back to it. Just make sure teh staples are spaced enough so the pieces can't fall out.
then you can layer several of these and they will be ready for you when you are ready to pick them up and work on them again.