I bought a pinch weld jack adapter before reading about using a hockey puck. The adapter looks like this:
Grip Pinchweld Jack Adapter
Hockey puck is cheaper and doesn't deform like how I initially feared it would. If you use a piece of wood, make sure you cut the groove across the grain (not with the grain). Also watch that the piece of wood doesn't compress (soft wood)...a piece of hardwood would be better.