I would consider ditching the tank bag idea, and goign with saddlebags instead.

I recently got some of these
http://store.proitalia.com/108460.html

for my 1999 900ss, and they work *great* - tight, flow right into the lines of the bike, and have a cool semi-rigid design that I really like.

You can get them with matching-colour zippers - the fashion police would approve