bstie,
I've got the same rattle when I go over bumps at slow speeds. You may have already read this thread, but in case you hadn't I started a thread about the rattle a few months ago and got some interesting replies. Maybe one of these will help.
http://corvetteactioncenter.com/foru...ad.php?t=36545
I've tried adjusting that rubber bumper but had very little success. The one thing that works for me (and its not very efficient) is to put a sponge (like one that you would use to wash the car) between the top and the deck lid. That usually keeps it quiet.
Good luck