It is said you can never have enough clamps in your workshop! However there will be times where you have a clamp, but it's not long enough for the job at hand. A clamp extender is a simple to make fix for this problem. There are many ways to make these, but here are some examples:
Both made from an of-cut of 19mm ply, with a clamping block glued and pinned to the ends. Different versions might suit different clamp types.
An even simple one made from some 4 x 2 with notches chopped from the ends:
For example, used to square up a cabinet after assembly, clamped across the "longer" diagonal: