Grab an eraser, then cut it down to the size of the head of your hammer. The thicker the eraser, the more cushioning you'll have. After it's been trimmed down, attach it to the head of the hammer ...
It's purely the melamine foam that makes the eraser work as well as it does. You can buy the foam as pre-cut rectangles fairly cheaply online but I like that the Magic Eraser is ergonomically ...