I instead had to create jigsaw-puzzle pieces of metal that would fit the holes that they are to fill pretty exactly. These patch panels are going to be “butt-welded” into place. The piece of metal fits the hole closely enough that once welded, it fits the hole exactly enough that the weld spots need only be ground flush to the body and then filler spread in to fill any irregularities. A really good patch panel that is butt-welded in could be so precise that little to no additional bodywork is needed.

Since I’m doing this for the first time, I’m betting that I’ll be miles away from a seamless weld, so have a can of body filler ready to go.

I got a flat piece of steel.

I have a car with compound curves in the places that I need to weld patch panels in.

How to get the steel to fit precisely before welding starts?

There may be a better way, but the solution that I came up with on my own was to use masking tape. It starts as a flat plane and can be taped over the channel. I then use a utility knife to slice through the tape, running the blade along the outside edge of the metal. The resulting piece of tape is an almost exact match of the metal. When the cut tape is pulled off of the car and stuck onto the sheet steel, it has a funny curve to it, but this is the curve that the steel will need to fit exactly.