So I'm going to show you how to make your very own microwaveable heat-up bad in under 10 minutes if you've got access to a sewing machine.
First of all take a cheap length of some cotton fabric and cut a rectangle out of it. Size doesn't matter, you can chose to make a big bag or small bag.
Step 1: Sewing It Up
Fold your rectangle in half and sew two of the three open sides shut. This leaves you with a handy dandy sack for your rice. But that's not all there is to it, first reinforce the sewn sides with a zigzag as you don't want rice escaping. Then turn the bag inside out.
Step 2: Filling the Bag
Make sure your stitches are on the inside (unless you want that rugged look :) ) and start filling with rice. Just the cheapest rice you can find, mine was about 50 cents for a kilogram.
I found that the easiest way of sewing the bag shut without spilling rice everywhere is to pin it shut, then support it with one and as you sew the last edge shut. Zig zag that one too and voila, your heap-up bag is ready. You can pop it in the microwave and start enjoying it's warmth right away or you can continue with the next steps and make a nice snuggly fleece cover for your bag.
pop it in the microwave for somewhere between 1 and 3 minutes at full whack, depending on the size and the temperature you want it to be. Be careful when you take it out, it can be very hot.
Step 3: The Fleece Lining
Lay your bag on top of your fleece (mine is a ikea blanket, I use them for everything) measure how much you're going to need (about 2,5 times as wide as the bag). Fold it it and sew it shut on two sides with a zigzag stitch as it is more flexible and will alow the fleece to strech. make sure to leave the extra half unsewn as this will be the way the bag doesn't fall out. Think Pillow liner.
Step 4: Almost There
Here comes the only tricky part. Fold over the edge of the flap if you want the liner to be nice and tidy, or if you don't care how it looks ignore this. Stitch the hem with a zigzag and pin the flap down to the main part of the liner, leaving the side you just hemmed free.
Sew that with a zigzag and then you're done. Stuff your bag in and admire your work.
When you heat up the bag be sure to remove it from the liner! Fleece could melt and give you nasty burns.
Have fun with your heat bag!