I'm pretty sure strapping a rubbermaid daypacker on top of the back seat isn't such a great idea. Something about having easily over 1k worth of gear strapped on to the rear seat with rope going down the interstates says bad news... Though I can't quite put my finger on it.. =)
Been doing it for 5 years now. All dive sites I use are at least an hour away on the interstate, though our speed limit is about 10 miles an hour less here than in the US.
When I have light loads, I use bungee, heavier loads require strapping with the tightening hardware (forget the name now, I'm forgetting my English).
We have a lot of delivery services that use motorcycles exclusively here and they use a similar setup, but with a locking case. For more security, you can put a rack that would be a more stable platform.
Surprisingly stable for me, though. I went down near my house once when we had 10 cm of snow and the daypacker stayed in place. Unstrapped the daypacker, set the bike aright, then strapped it back on, and got back on the road....
Since we have so much traffic here, taking the bike to dive sites is a really conveneince when I don't need to take other people's gear, or other people...