Thanks to Julie for this Frugal Baby Tip!
I had a Kushies Flannel doubler. I'd actually call it a washable liner, because it's not that thick. But I traced it onto a sheet of tissue paper and used that as my pattern. Then cut out a piece of terry (old towel), and flannel (old receiving blanket) and zig zagged the edges together. You could also free hand a pattern, just make it a nice oval shape, large enough to fit in your diaper.