On a bluebird they have the integrated window "shades" over each section - I used that as the foundation for a continuous drip rail. The installation was pretty simple - a rivet at the beginning and end of each drip hood section, and sealed between the rail and the body of the bus with sikoflex.
When I painted elastomeric coating over it, I ran the coating right up to the edge of the drip rail which seals it further.
After it started raining I had to go back and touch up a few spots I missed with sikoflex one more time, but it's worked pretty fantastic at keeping the rain off things.
The drip rail is galvanized so it's fairly resistant to corrosion.
It also has a secondary purpose that's not obvious except with close inspection. When I installed the new sheet metal on the bus, I ran the metal up inside those window eves a bit, but I intentionally left a gap. This gap acts as a ventilation space to allow airflow between the layers of the wall inside the bus. I have found it to be extremely effective, and act exactly the same as soffit vents. The trim hides the venting feature and keeps water from intruding.
If I damage the rail it's very common stuff: Construction Metals 1-1/2 in. x 1-1/2 in. x 10 ft. Galvanized Steel Roof Edge Flashing-RE15G - The Home Depot