Thanks for the tips -- I started to put it together the way you suggested, but the spring wouldn't compress enough to make it fit that way, so I don't think that is quite how it works. However I followed your advice and tried to align the marks left by one part on another, and I think I figured something out. This doesn't seem to be quite the right arrangement either, though -- now the screw doesn't stick through far enough to attach to the fuel cap.
Still trying, though. I'm surprised there's no diagram anywhere of how this works! I found one archived post here that looks like it would have been helpful, but the relevant image was missing...
[edit] nope, that arrangement doesn't fit either. Hrm.