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.