I always try and pass on any gizmo that works for lower back pain. I found this one on Dragon's Den: the Acupad, developed by a chiropractor, Dr. Cohen.
When you first handle it, it feels hard and bony, but 1½ minutes in the microwave not only warms it up, but makes it softer. And it retains the heat for a long time. It fits snugly in the small of your back and the heat really soothes the pain. Now I have got mine, I carry it everywhere. So I arrive at a friend's house with it already heated to go – and the heat will last.
The instructions show other ways of using it/them, but I just love it on my lower back. Since having it, I do seem more mobile and less fraught with pain.
Dr. Cohen also makes an Acuball and a sort of dumbell called the Acuback, but they are too large to work for me. The joy of the Acupad is being able to lie down on it and let the heat seep into your bones.
Acupad description – link
Acupad review – link