I may have a solution for you, but we don't have any roof vent covers that have the side slide that you're looking for and I can't say that I've seen anyone successfully modify these since the arms are riveted into the plastic.
With that said, you could use the Camco Aero-Flo RV Roof Vent Cover w/ Swing Open Lid - White # CAM40421, which basically acts like 2nd cover and would allow your roof vent to remain open while still preventing rain from getting in. This solution would allow you... view full answer...