I use heated bean (or rice) bags. That way I get to control the amount of heat by location. If you want 'wet' heat, soak a towel and then wring it out. Put that closest to your body, then something waterproof like saran, then the hot bean bag on top.

To make a bean or rice bag - take an old sock (no holes!) and fill about 1/2 to 2/3 with any kind of uncooked bean or whole rice. Tie it off and heat in microwave to desired temp. I usu. go for 1 minute or on high to start and go from there. You can reuse over and over if you keep the contents dry.