What is this exterior wood finish?
I would do the following, assuming you have some scraps from making the panel:

Print out the photo and take it, along with a small scrap, to your local store with outdoor paint stains.  (big box, hardware, or paint store) and see if you can match the color.  If you are lucky, a person there might be able to narrow down the choices based on the type of wood. Purchase a few of the smallest cans that might create the same results.  On some of the scraps stain them with the different stains and let them dry.  Compare them with the original stained piece.  If they are not close enough, repeat the process.

If you still end up with differences, try getting it close and stain the new part of the door.  Then apply a covering type stain over both the old and new.