I did find that a microwave just wasn’t the thing, unless it still did not have the tubing on it. If just a bag, the microwave will work — keep jiggling it every few seconds and feeling it. Once the tubing is installed, the microwave will rapidly cause the fluid in the thin tubing to boil and the plastic to melt, ruining the tubing. In the fridge the fluid bag with tubing was stored (I kept it in an empty crisper tray) and then to the heating pad it went a couple hours before use.
Do I even have to refrigerate the bag at all? Right now it is just sitting on the counter. Maybe it is ok just at room temp. With air conditioning on.