Making the assumption you are trying to avoid the cost of the real thing?
wool batting, quilted so it didn't shred over time?
wool blankets and throws are readily available.
all that said, I had one, specifically sold as a mattress cover (light and breathable) and it consistently slid and wrinkled under the sheet. Uncomfortably so. I added four more elastic with clips to sides and top/bottom and it didn't help.
I suppose you could add one to a standard fitted sheet and it might stay in place that way?
I purchased and struggled with mine because it was recommended to keep our thermopedic mattress from molding.
I loved that mattress to sleep on but it was a struggle in many ways, and I'm glad it is gone now...