Is flannel necessary for castor oil packs?

Organic castor oil is very expensive and I have found that even soaking a piece of flannel, the castor oil seems to dry up very quickly. I have been putting it on my tummy then covering it with a towel then applying a hot water bottle.

I just read about using a damp cloth in between - I will try that next. But do I have to use flannel? It seems like to properly soak a piece of flannel it would take half of the bottle and the bottle cost $20.