I pulled out two unfinished pieces from a class that I taught last summer. They were almost the right size--all I had to do was find a few 2 1/2 inch strips to add as borders. I auditioned several and finally settled on a yellow and a stripe.
Richard wanted me to use the original cover because, he said, it is treated with fire retardant. I said that any chemicals it once had are now long gone, but I didn't argue: in it went. And, in fact, I used it for a pattern.
A piece of muslin backing, some quilt batting, the original cover and the pretty tops, and we were in business. I stitched the front and back pieces together along the bottom to make one piece. It didn't take long to quilt.
After quilting it, I folded it in half along the bottom seam with right sides together and stitched the side seams. Then I stitched along the top edge leaving a space open against the corner. My particular heating pad has the cord coming out from a corner. I used the open space to turn the cover right side out, then hemmed the raw edge with a small piece of binding that I sewed down right side together and turned into the opening.
That's it. All done. I really should have done this long ago. It was much easier and faster than I imagined it would be. That said, if you have something small that you want to cover, get busy with it. It doesn't take very long.