A HOME MADE CAT HEAT PAD
This is based on information for making a reusable heat pad for humans.
Making the Heat Pad
Cut a rectangle of calico (or similar strong fabric) about 70cm x 25cm. Fold this in half to give a rectangle 35cm x 25cm. With right sides together, hem the sides together leaving an opening about 8cm wide. Turn the bag right side out.
Fill with about 1 1/2 Kg of whole wheat (from a stock-feed merchant or poultry feed store). If desired, add a handful of lavender and a few broken cloves. If you use other herbs, test the microwaveability of them first i.e. can they be heated or do they go up in smoke. Do not use essential oils before or after heating - many essential oils are toxic to cats. Some cats dislike strong herbal smells so only use herbs if your cat likes them.
When filling the pad with wheat, make it fairly firm so that it contains enough wheat to retain heat for a decent amount of time and so that it can be moulded to shape (it can also be used around human elbows, knees etc!). When the bag is sufficiently full, close the hole with secure stitches. Do not use a zipper - metal zippers will damage the microwave and plastic zippers may melt or give off fumes.
If desired, make a removable washable cover for the heat pad. Otherwise the heated pad can be wrapped in a towel or placed under the cat's blanket.
Using the Heatpad
Put it in the microwave, and heat on medium-high or high power for approx 3 - 3.5 minutes. The heat pad may feel a little damp after heating - this is normal and soon goes away. Sometimes the heat pad can get a little too hot (microwaves vary) so test it by squishing it between your hands to release some heat. Make sure it is at a pleasant temperature before putting it under your cat's blanket.
The wheat-filled bags are at their best for about an hour, but will stay warm for 2 - 2 1/2 hours. It is also possible to heat a wheat filled heat pad in a conventional oven.
It is advisable to place a cup of water in the microwave oven with the heat pad to prevent the wheat drying out too much and possibly catching fire. This also protects the microwave unit, as this operates on the liquid content that is being heated, and can be damaged if there is insufficient moisture in the oven. It is advisable not to reheat the bag until cold, and not to heat it for more than 2 minutes.