I wonder why they stopped using the long one in the door. The panel has screws that go into metal in that area, so it would make sense.
anyway, I will seal them. I probably don't have linseed oil, but I do have tong oil. Which for a part that is never seen, should work well. If you need to make some pieces the tools are simple. table saw, bandsaw and belt sander. You could substitute a saber saw for the cut out.
I made news ones because there were so many holes in the old ones. I can't imagine how many times the panels were taken off to get that many holes. And still, when we got the car, there were no panels.
Jerry