Thanks for all of the suggestions. I did plan to sand the seam down as much as I could, but there are little gaps here and there, and I wanted to get it all evened out. I had thought about using FNF, but I didn't think it would stick well enough or be hard enough. I do have some Aeropoxy Lightweight Filler, which I have used with much success on balsa parts. It is a bit pricey though, so I only use it for major dings and salvage projects. (And for awhile, I forgot that I even had it. :blush: )
I used some Squadron Green Putty for the first time last fall to fix some spots on styrene parts on the Saturn 1B that I was building for the Apollo 7 Celebration. I was a bit dismayed at how soft and crumbly it was when I first applied it, but after it had enough time to cure really well (about 24 hours), it performed OK and did what I needed it to do. (Can't ask for much more than that from any product, actually.) It didn't stick all that easily to the styrene at first, which is why I wondered whether the white putty, and not the green, was more appropriate for plastic. Now I can't even find that tube of Squadron Green - maybe I tossed it.
(Nah, that couldn't be it; I never toss anything...)
I thought about using Bondo, too. I was worried that it might cure too hard for the styrene (making it difficult to sand without damaging the plastic). I have never actually used Bondo yet, so I didn't actually know what it was like to work with. The hardness issue was why I rejected another idea that I had: spot filling with epoxy cut with microballoons.
All of your suggestions are telling me that this problem isn't as big of an issue to resolve as I thought at first, and that there is more than one way to skin a nose cone. Thanks so much.