Interior fresh air doors...and Go!

In response to Trebligs comments, I don't think completely sealing the cowl is doable. To close/delete/cover the exterior grille would keep the leaves, pine needles, and such out but not water, dust, pollen. There's a crowd of other small openings into there.
As for the fresh air boxes, if someone would create covers, variable vent w/ screen to replace those swinging doors, put me down for 2 pair.