Here's what I would do: Find get a hole saw slightly larger than the curve of the light. Get a piece of 1/4 plate. Layout the light, use the hole saw to cut the curves, use a juig saw to cut the straight sections. Clean it up with a file.
Lay out the light on the fender, Leave 3/8 of material to the inside of the line, cut the fender. Take the 1/4 piece and clamp it behind the opening. Use it for a forming block and hammer
the metal into the opening in the 1/4 plate. You may need to curve the plate slightly to contour it to the fender. And your hits will need to be exact to get the panel to stretch it without mis-forming.
You'll end up with a nice finished edge around the light and when you cap the back where the light will sit the fender won't warp. It will take you 3 time longer but if you do it right you won't need body filler. I don't like filler abuse.
But that's just me.