If you want diy I'd go with a piece of smooth or diamond plate aluminum. Look at it and imagine it was designed to go there. I'd probably goop it and then drill and put the screws but in. Or get stainless screws. Just use the original screw pattern so the aluminum is clean without even more screws in it.
Now here is the important part...you both need to get your story straight so that when people say why is that plate there you tell them that it was that way when you bought it. Or tell them it was a special recall that you had done that reinforces that area. :)
