I bought a 2 meter length of the foam insulator used to cover water pipes and cut it in half lengthwise then wrapped it around the plastic rear shelf to create a better seal. Tuck it down between the body and shelf. I then wadded up some bubble wrap to create a block in front of the leather hinge and covered it generously in duct tape. The supplied tool bag sits in front of it to ensure a good seal. All of this really worked in keeping the contents clean and dry. The seat belt channel is a big problem in that the belts are in the same compartment as the rear tire and get exposed to dirt, mud and just plain water. Still trying to figure that one out.
Taping the seams behind the seat cushions will help a great deal but the access to the bevel box will be constantly removed so it will have to constantly redone.
Did all this but...
A great deal of dust will then come from the front end. The transmission tunnel is the biggest culprit. The dynamics of the cockpit create a partial vacuum there so that very little comes from the top but it does get sucked in by any available openings elsewhere.
Avoid dusty roads.
