Natural, unbleached flannel would of course be the very best thing that you could use - no dyes or chemicals. Flannel is used for castor oil packs for 2 reasons - heat retention, and the nappy, almost fuzzy texture of the fabric grabs and holds oil better than a smooth fabric would, and more of it. Another thing I've noticed is that the flannel seems to cling to my skin better. Not to say that you couldn't use a diaper - sure you could. If you're putting a hot water bottle or a heating pad on it, the heat retention of the fabric is not really all that important - it's going to warm up. And the amount of oil is not really a concern either - if you use a diaper or smoother cotton, and feel that you've absorbed all of the oil, then simply add more.

I get my flannel via an old and much-loved beige flannel sheet that has seen better days. I cut squares off of it.

This thread has inspired me to order more castor oil!

I had dark circles under my eyes since childhood and I read a book on face excersize.
Stare at the ceiling and try to close your eyes using the botton lids only. This improves the circulation and helps with blood pooling.
Feels really weird at first as we don't use these muscle much but it worked for me and costs nothing.

Regards
Helen