Depends. Some side of a cupboard or rack with flexible enough sidewall (heavy enough or bolted to the wall to not fall down). A simple desk with resonating plate, with a cushioning leather bag or rubber pad (dickes Filz-Mauspad). A plastic garden flower bowl filled with clean (not rotten) sand.
Some special constructions I won't tell you ...
Originally made of animal leftover.
Don't strike hard, just drop the hand.