A guy could weld a plate over the top of the inner-to-outer cowl panel to close that up, but there would still be a gap after the inner and outer fenders are bolted on. Like has been said, the rubber is there to fill the gap. The filler plates can't be spot welded on after the kick panel is installed because there wouldn't be a way to get the electrode behind. But they could be plug welded, but the rubber would likely be melted in the process. Thus the screws.
