Cutting an old quarter panel up is easier of course, but you probably can't find one for a reasonable price. 4speeds makes a good point about the contour of the wheel opening. This is how I make them.
-Make a cardboard template of the wheel opening before you cut the rusted piece out.
-Cut out the rust....straight lines are easier to patch.
-Cut your patch.......make it lots bigger than the hole, bend the lip
-Clamp it on the car in whatever position is required to maintain the contour of the wheel opening.
-With the patch clamped in position, reach in behind with a pencil and trace the hole opening onto the patch.
-Remove the patch and cut along the pencil line.
-Tack and weld.

Take your time and remember......you can't make it worse.

-jr