Rain on the front element
Two hours facing into wind is a different problem from walking about in drizzle, and almost everything sold as a rain cover is designed for the second one. They are soft, they flap, and the flapping puts drops exactly where you did not want them.
What works is rigid and boring: a section of 68 mm downpipe, cut at roughly thirty degrees so the long side is on top, taped to the lens barrel with gaffer tape. It extends about 90 mm past the front element. Wind-driven rain has to turn a corner to reach the glass, and mostly it does not bother.
Two things I got wrong first. Taping to the hood rather than the barrel — the hood twists off under its own weight once wet. And cutting the angle the other way round, which funnels water inward with impressive efficiency.
Condensation is the other half and the pipe does nothing for it. The only thing that has worked there is refusing to bring the camera indoors between sessions: it lives in the boot of the car, at whatever temperature the night is, in a bag with a lot of silica.