Please don't use rice. Been there done that. When it gets old it STINKS it get rancid. I went to deer corn but then after several years its gets stinky too. Now I use flax seed. Just be careful not to overheat. It can start a fire. You can add lavendar buds to the heat bags. Scented oils won't last in the rice or corn.
Fleece is too heavy it doesn't let the heat come through as well. Flannel or cotton is best.
I have made so many of these and sold a pile of them. I was just sorry I used the rice when I did.

We keep the house pretty cold in the winter, so a few years back I made some for keeping hands, beds, feet, etc. warm. They are 2 pieces of 10"x8" plain white canvas, stitched together (cotton thread) on three sides with a 1" allowance all the way around. Fill the bag with wheat berries (smells like bread baking), tuck the open end in and stitch closed. Before using I microwave for 3 minutes (2 or 3 times, cooling in between) with a half cup of water next to it to remove some of the moisture so it doesn't mildew. I've never put any oils or dried herbs in them,so I don't know what to tell you on that. At the end of winter I just dump the wheat berries in the compost pile, wash the bags and start with fresh next year. Hope this helps :)

Here is a page of info for you.

Here is a link that might be useful: making heat bags

these are WONDERFUL! one year I made gift baskets with these in them and they were a huge hit. I use mine all the time.
kudzu

" Before using I microwave for 3 minutes (2 or 3 times, cooling in between) with a half cup of water next to it to remove some of the moisture so it doesn't mildew. " Honey - putting a bowl of water will not remove moisture - it will add it. Microwaving the bage will assure thate it wil not mildew unless you are somehow getting it wet. If you live near a feed and grain store, get some field corn - it heats nicely and is actually washable! You can hand wash it in the sink to refresh it.

The water allows the bags to dry out slowly over time without the possibility of igniting. I've been doing this for years without negative effects. I tried some of DH's feed corn and wasn't happy with the size of the grain. Everyone has their individual preferences, but thanks for the advice :)

I see a lot made with black eyed peas,too. my rice ones have held up for years now, I guess we don't ever notice a smell?? they've never gotten dirty so I haven't had to worry about washing them yet. I've heard many many people suggest flax seed.
kudzu

Here is a link that might be useful: Kudzu's Blog

I also use rice,it's soft on the body and I think it keeps warm longer. When you nuk-it, shake between the 2-3 times. Next Spring plant lavendar, sage, or plants that offer the scents you like...rose petals. Go Green.

I started out using wheat but it is more difficult to find than rice. Perhaps the rice bags might have been overheated. I'm sure the cup of water does keep them dry as I use it to microwave and dry flowers all the time. Guess it is a science mystery why the water removes water from the flowers or rice.I also used turkish toweling for the bags.