So I added this to another thread, but I am re-posting here, anew.
I have an aftermarket headlights and aftermarket front bumper (from an accident 2 years back), so the seams aren't exactly "tight". So while at Menards I saw something that may work, flex PVC. Seen below. The size purchased will depend on the gaps that are being filled, but I picked up 1/2" OD, cost ~$2.00 for 10'.
And here's the gap to be filled, upper left of the headlight.
Now the nice thing about this PVC, is that is it is quite mailable and can be manipulated to fit the gap. I squeezed it then pushed it in (the warmer it is the better), filling the gap. Then it expands for a tight fit.
Nice, clean and cheap!
Now if your car isn't black like mine and you are worried about the color, pick up the clear type, you won't see it as much.
Word of caution: These may fall out in the "auto" carwash, or at high speeds, depending how tight the fit is. Also as mentioned above it gets soft with heat, so be aware of that as well.
-Dave