The "Thai Crystal" product is the right stuff (Potassium Alum). This is what it looks like (Amazon image):
http://ecx.images-amazon.com/images/...160_AA160_.jpg
Comes in other forms but this one is nice because the plastic base makes a nice handle.
I stand it on its head when not in use so any water in the base will drain out.
I also use it for its intended purpose as a deodorant. Should last about a year.