I have removed some of the old weatherstripping. It has the same cross-section as the new door weatherstrip. However, I don't think it is correct or stock, because the gap shows air all around.
So I called Steele Rubber, and they provided a great suggestion: use some modeling clay; shape into a cylinder larger in diameter than the gap between the clamshell and the body; wrap in plastic wrap, feed into the gap, and shut the gate. Then cut the end of the clay with a sharp razor blade. The resulting shape should be a very good exact fit.
Anybody ever try this?
