When looking at this question on another board someone mentioned the Givi E30N top case. They even showed some photos on their K12GT, and it looked O.K. because it was a smaller case. Plus they are only about $85 mail order, and supposedly their base mounts directly to the luggage rack of the K12GT (although you might need to get some longer SS bolts).
If looking for a hard case this was definitely something that I was going to try.