This may be only mildly applicable to this situation. Instead of mushroom vents, I installed clam shell vents. Ordinarily these are used on boats. I purchased mine from here: http://www.iboats.com/Seadog-Clam-Shell-Vent/dm/view_id.886137
They’re stainless steel, so no UV deterioration of plastic. And no moving parts to wear out. I did add window screening underneath to keep out insects. And I fashioned plugs from packing foam to seal out drafts. An application of RTV silicone around the inside edges made an excellent gasket.
Best of all, they have not leaked at highway speed and rain.
