very cool, Miss HF. I have shied away from doing castor oil packs because of the mess involved, and the complicated ritual...if I remember correctly, soak flannel in castor oil, cover with something(or maybe not) and then cover with heating pad.
Looks like you have solved those problems sweetly. Removable washable inner liner, microwaveable outer(yes we still have one too, although I am not sure why, it's gathering dust, the fact is most people have one, admit it or not).
Have you thought about marketing these?
I would most certainly buy a plain one(as in, not necessarily meant for a particular body part). Maybe rectangular or oblong.
Hot dang that would be an excellent way to hasten delivery of essential oils to the system as well.
And, you could make the covers in cute fabrics. Geisha girl or cowboy for me.
I'm serious. Make me one, NOW. I'm planning on languishing on the couch soon and I need accessories.