I've done it on my FZ1. There are details on
my blog for how I did it and
here are how some others have done it on bikes with a separate pass seat.
One guy did it for his ninjette and built a full wooden base.
The case is pretty stable. My seat only has the one latch in the front with nothing for the rear, but the case is pretty stable. The only time it moves is when you shake it and the seat wiggles a little. I'm pretty pleased with mine except that I wish I could have mounted it straighter, I noticed it's a little to the left of center.
I plan on lowering the front seat and when I do I'll address that but for now it's perfect.