Go to a hardware store and get the size bit you need that is made for metal. Most cast iron isn't really hard unless it is white cast iron. If the hole is over 3/16, pre-drill it with a small bit first. It might be a good idea to use some cutting oil to.
For more info, see the section on calcium absorption. High calcium foods include dark leafy greens, cheese, low-fat milk and yogurt, bok choy, fortified tofu, okra, broccoli, green beans, almonds, and fish canned with their bones. The daily value (DV) for calcium is 1000mg.
Other food sources are more absorbable like bone broth, dark leafy greens, fish with bones, and even carrots. Some foods like spinach, which is often suggested as a good dietary source of calcium, are only 5% absorbable, which make them great for other nutrients but not a good source of calcium. Bottom line: We need about 300-400 milligrams of absorbed calcium per day from food sources and not much more than that.