Back in the days before all the fancy fusibles, we used dryer sheets. Used ones. They worked great. They were soft and pliable. They stayed in the applique. You had to clip a hole in the sheet, sew it onto the right side of the applique, and turn it inside out through the hole. If you preferred, you could trim the center of the dryer sheet back for less bulk. This turned the edges under, but it got really tricky with appliques that had intricate profiles. (Those are still tricky for me!) The dryer sheets only came in small sizes too. I still do it this way occasionally.