Hi Chuck,
Forgive me if I'm suggesting something stupid but really hoping to help you find a solution. I'm looking at the photos of your tail light section and from the photos, it looks like the steel comes to a crest in the middle. On the GS' I've owned, the middle section of the steel is more flat and not as pronounced as yours. I read back in your early posts and you had lots of mastic to be removed and maybe that compensated for the crest. I'm thinking that crest is causing the tail lights to splay out and not allow the emblem to fit down properly. Your work is impeccable so I hate to even comment without being there in person. Hope that helps.