Not so easy to make your own here. You'll need to do some experiments with what you have access to.

What you need is something that can be heated in a Microwave oven yet not burn up the oven. Then you need to find something that hold the heat for a while and does not make a mess or stain on your iPad.

Frankly, by the time you figure this out (if you can replicate it with what you have access to). It would be easier to just get one or two. Sorry ;-{