What are the ingredients in a microwavble heat pack? I would like to make one !

What are the ingredients in a microwavble heat pack? I would like to make one as a gift for a friend.?

I know some are filled with rice or flax seed and herbs such as mint or peppermint, but I'm not sure of proportions.

Contrary to popular belief, you can make your own. Typically, a microwave heat pack is several layers of muslin with long grain rice in it. Note: do NOT use minute rice or any other quick cooking rice, as it will burn in the microwave. Ordinary rice will work, but I find long grain rice holds the heat longer. You can use flax seed, but I find rice is cheaper and doesn't add its own smell to the mix, like flax does. Flax (linseed) puts a varnish sort of back odor (linseed oil is used for oil paintings as both a painting medium and as a varnish) to the pack that I find objectionable in some cased.
As for proportions, experiment. Start out with small amounts of herbs and work it up to your tollerance level. At first, you can leave one small opening in your pack until you get the right mix of aromatics. Then seal it once it's where you want it. I find about a tablespoon of most herbs works well, but you have to be careful on really strong smelling herbs (notably, rosemary) and use about half as much to avoid being overwhelmed by the odor. Size and shape depend on where you want the heat. Shape to fit the area you want to warm. If you get good at making these things, you may have a source of extra income. Think: flea markets, yard sales, and local health food stores (where you might be able to put some up on consignment at places that don't want to buy them outright for retail).