I watched a 'howcast' on how to make your own heating pad (http://www.howcast.com/videos/212549-How...). Like some of the above comments, rice and cotton cloth were the primary ingredients. My problem with the iOpener is that you can only heat 1 side at a time. From an engineering standpoint, a device that would cover the perimeter of the device would be the most effective.
So I took these few basic principles and went to work.
Step 1: Steal wife's sewing machine
Step 2: Put on hat and dark sunglasses and go buy some cotton fabric (the plainer the better)
Step 3: I personally took precise measurements for this, but you could just make a 90" x 4" rectangle and fold it hotdog style
Step 4: Sew 2 of the 3 sides together
Step 5: Get your hands on some "deer corn" (but rice works too)
Step 6: Fill you sewed tube with the deer corn and sew shut
Step 7: you're done. go grab a beer and tell your wife to clean up your mess.
For those without a sewing machine:
Materials - pair of tube socks, deer corn
Step 1: fill tube sock with deer corn
Step 2: tie tube sock shut
Step 3: repeat steps 1 & 2
Step 4: you'll never get to step 4 unless you skip step 3