Instead of cutting out a template from a piece of cardboard, use a blank piece of paper, put it over the hole and use your dirty fingers to trace the outline of the hole along all of the edges. This way, you have an exact outline of the piece of metal you need to cut out to fit into this hole.

I decided to cut away a much larger piece of metal because it would have been just as easy to weld in a large piece as it would have been to weld in a small piece.

I also decided to make this a two-piece repair because it makes it a lot easier to fabricate the pieces. Less metal bumping.

Welded ...

... and ground.

The area is covered with a thin coat of body filler.

Same thing on the other side. There were just a couple of small rust holes but I decided to cut out a much larger section.

A new piece of metal is fabricated and ...

... welded in place.