Hengelo, Nick is talking about the opposite, a punch with a concave surface. I just bought a cheap pin punch set and using a burr in the rotary tool, carve different size punch with different size round depression at the end. It helps a lot in keeping the punch on the pin while you hammer. Nick also posted a jig for holding a knife especially for this purpose. Like he said, having the knife solidly attached is a must.
*Sorry, I am a slow typer*