There are 3 layers to a castor oil pack – a flannel or soft cloth soaked in castor oil, a piece of plastic and a heating pad, in that order.
They say a wool cloth is better but cotton seems to work fine. Castor oil, as I said in the Salt Scrub post, is nicknamed the hand of God because it seems to reach into the soft tissue. It is also a powerful laxative as you know, but not when applied to skin as far as I can see. There are various versions, including cold pressed, but any will do. Multiple applications my be necessary.
If there is no heating pad available, try heating the soaked cloth in a double boiler or microwave oven first.
Castor oil is heavy and hard to remove from fabric. I keep an old, oversize t-shirt to wear when I use the pack.