It seems that the bumperettes must be attached to the trunk lid. I can't see any other possibility.
Yes, I have more than mixed feelings about this, too. I don't like the population of Sprites diluted by "replicas" (which, if I were less kind, I could arguably call "fakes"). Beyond that, however, I just don't see the point. There are plenty of real bugeyes in the world, and it's probably no more trouble to get a real one in decent condition and restore it than to build one of these replicas. I guess for some kinds of cars, someone might want a car that looks classic, but has modern, reliable components. But that doesn't seem to be the case here.