Amazon offers Porter-Cable Corner Chisel for . Eligible for free super saver shipping. 

Product Description:
A must-have for hanging doors, this spring loaded corner chisel is more efficient than a hand chisel, and works quickly to ensure that corners are perfectly square. Compact enough to carry in your pocket, with a 3/8-inch cutter edge, this handy tool creates a 90 degree square corner when simply placed on any corner and tapped with a hammer. And because the Porter-Cable 42234 Corner Chisel self-aligns, accuracy is never an issue; you get the right fit every time with minimal maneuvering.
Features:

• Used to square the corners of hinge and door lock recesses
• Spring-loaded; 3/8-inches cutter edge for precision
• Self-aligns for optimum accuracy