There are inexpensive muslins available.
Or you could buy the cheapest (least expensive) sheet you can find and that would work.
Even hand washing just a top is an iffy process. I had to wash some sections of a top (one of my cats "anointed" it - thought I had everything away from the wretch) - and there was a LOT of fraying. This was just dunking the sections in a sink and hanging them on a drying rack. Also, some of my stitching came out and I had to redo some of it.
Warm and Natural batting machine (automatic) washes (warm or cool wash and gentle cycle) and machine dries (permanent press cycle) just fine.