After watching my mom and sister spend $12 a pop on those store-bought disposable heat pads and around $30 or more on various gimicky microwaveable reusable ones, I stumbled upon this idea while marveling at how MUCH pee my nephew's diaper held.
A bag of 40 diapers costs about $10, so you will be spending about $.25 each. You can reuse these a couple dozen times easily, so figure on a penny per use (not including any energy cost for running your microwave, of course). Not bad, when compared to the $12 for one or two single-use disposable hot pads at the drugstore. These retain thir heat for a couple hours.
Step 1: What you need:
A clean, dry disposable diaper and a good sturdy freezer bag.
Step 2: Open the diaper.
Open up the diaper and spread it out so it is like a cradle.
Step 3: Fill with water.
Take the disposable diaper (preferably clean) and run it under water, pouring the water into the hammock-like center until it is puffy and full. If you are interested and/or obsessively inclined you can measure one cup at a time. I found three cups is perfect for my nephew's size 4 Kirkland diapers, I doubt there is much variation needed.
Whichever method you use, try to get the water absorbed to all edges and press it around a bit so that it is nice and puffy end-to-end.
Step 4: Enclose in freezer bag.
Next get a gallon-sized heavy-duty freezer zip bag and position the diaper neatly at the bottom. Squish it around to make it perfect if you want to. I do.
I fold the outer edges inward to form a neat tubish rectangle and then stuff it into the bottom of the freezer bag. Again, squeeze it around and work it into the shape of the bag.
Step 5: Heat it up!
Now place it in the microwave for about three minutes and thirty-three seconds (depending upon your microwave's power). Make sure to leave the bag partially unzipped or it might explode. Be careful while removing it, the heat can sneak up on you in spots!