I just did this on Natalie's 525iT in the Spring. Go ahead and buy the clips, you will have to trim off the bottom edge with a set of klines or tin snips, leaving part of the "C" shape in place to fit under the window. It takes a little manuvering, but they will work into place, provided its a little warm. Be careful not to pry too hard to seat them on the widow edge. The double stick tape they come with will further help to keep them secure. Do about 24-25 of the little buggers, and snap your trim back into place knowing it is secure. Do them one at a time as to locate them because they correspond to the openings in the trim. It has worked now for the past 8 or 9 months with no issues. Just like you, she heard this banging when she passed a truck, she pulled over and saved both pieces, even the little center piece.
It's not that bad a job, takes about an hour, you might buy a few extra in case a couple goof up on you.
Hope this helps Scott,
Jr