3/8" Spot weld cutter from Maxx autobody supply. $10 Canadian

The pin in the middle is heavily spring loaded and serves just to center your drill over the spot weld until the holesaw-like outer teeth are brought to bear on the metal, as you push down.

Here's a shot of the cutter without the hole saw-like part on. You can see they thread on.

A shot of the reversible holesaw-like cutter, which threads on either way for double the life. You can cut through just the outer skin of the quarter panel, so as not to leave the inner panel looking like swiss cheese.

Here's one in action. The truth is I find you have to pilot first with a small drill bit (7/32")halfway into the first layer of metal otherwise it wanders all over the place. That said I found these very useful and will use them again.