I have been considering doing the castor oil packs. I found the site below a few weeks ago which sells necessary and "nice to have's" regarding the castor oil packs. One such "nice to have" is a velco holder than keeps the caster oil pack in place. It is pricey at around $30+ but if you do them often enough, it might be worth the price to not have to hassle with trying to keep the pack in place or from getting the oil all over you.
The product information is as follows: Experience all of the benefits of a castor oil pack with the convenient aid of our wrap-around holder. Two velcro strips secure the flannel over your abdomen while an ample-sized pouch holds your heating pad (or hot water bottle) in place. Flannel-lined vinyl protects your heating pad (hot water bottle) and clothing from the oil so that you can simply relax.