I'm surprised to hear a few people have had problems mounting the Kreiga, I took mine away for a week with a US20, US10 on top of that and a Held tail pack clipped on behind, all mounted on the rear seat with no problems.
The rear seat itself is pretty weak, you can't just strap the US20 to it. I used the loops that come with the pack and mounted them to the bike under the rear seat, 2 where the grab handle is and 2 off the rear bolts that hold the fairing on. It's completely secure and the bag doesn't move at all. I also have the tank adaptor and have mounted a US20 on that with no problems.
I'm away at the moment, I'll post pictures when I'm back.