One great trick for making small patch panels like this is masking tape.
Here you can just see the outline of the rectangle I'm prepping to fill.
Masking tape over the hole and then razor blade it out as accurately as possible. Then since it's masking tape you can just stick it to scrap metal like a sticker. If I want to leave a gap and the shape is complicated I give the masking taped part a light shot with spray paint, peel off the tape and then bandsaw cut INSIDE the lines. The Paint won't lift like masking tape will as the metal heats. For simpler shapes you can get away with just using the masking tape.
The masking tape is also great if there is contour to the part. If your dealing with rusty metal before you cut it out, cover it in masking tape. Then scribe your cutlines through the tape. The masking tape is your pattern and when you try to force it flat it will tell you if you have a compound curve your dealing with. You can cut the maksing tape to flatten it and it shows you where to shrink, or anywhere the tape bunches you know you need to stretch. If the masking tape lays flat nicely you just have a simple curve and you can use whatever means to bend it.