Mine is shaped like a blade screwdriver, with a sharpened blade. It is short, about four inches long including the drawer knob. I put mine in the palm of my hand, extend my index finger so it can ride just behind the cutting edge, and start walking it corner to corner as I move across the surface.
If you visit the local saw shop, and ask for any worn out chainsaw files, you will probably wind up with a lifetime supply of graver material.
Hungry Horse