Yes to socks and rice. Use 100% cotton socks. Synthetics could melt, out-gas, or do something weird in the microwave.

The rice, or any whole grain, in a fabric sack or sock will work; as Russ points out the more that you heat it, the drier the rice gets and eventually won't hold the heat as much.

I've used homemade heat packs like these, and I've used the gel packs from the store, and I have to say that the gel packs hold heat longer and felt better to me. I think the gel conforms to the body more than the sack of rice.

Another alternative (which is probably not as safe as the rice in a sock) is to put a damp washcloth or small towel in a ziplock bag; squeeze all the air out and seal the bag. Then you can microwave it in 30 second increments. When it's hot, wrap another towel around the bag to insulate it from your skin and use as a hot pack.

Use an old sock for your rice. Tie a knot in the end and voila. My mother and my doula recommend them.

Why not just sew it in there? Then you don't have to use the rubberband. It could be an old pillow case you don't care about( or pick up at thrift store for a quarter) you only have to stich one side and it doesn't have to be pretty. Fill, folde the edge over so the grains font wanna come out at the seam and do a straight or slip stitch. Also the lavender mentioned is a great idea. I've had this kind of thing made from both rice and buckwheat both worked good ( smaller versions work for those fancy eye pillows too). If you go lavender use the flowers not the essential oil it can seep through the fabric and can be an irritant

I've heard of this done with dry rice and with buckwheat. Some might add dried lavender to make it smell pretty.

The dry rice heated in the microwave is certainly OK. No toxic fumes or exploding bombs that I can think of. My wife has used this with good success.

Zip ties never melted in my experience.