Here's another idea that I just remembered. Get yourself some of that thin green felt material with a sticky back. You can usually find this at hardware stores. Try placing various amounts around the tweeter, on the faceplate - not on the tweeter material itself. It should serve to focus its sound, but also provides a bit of damping, in my experience.
I tried a small piece on just the screwheads and it made a big change in what I was hearing. I've seen designs with entire rings around the whole tweeter. It all depends on what you hear, but I would suggest starting small and working up to where it sounds balanced to you.
Enjoy,
Bob