This galvanized steel vent cover is designed to fit over plumbing vent pipes and other small openings on top of your trailer. 

It fits pipes up to 2-1/4" in diameter and extending as much as 2-1/2" above the roof.  The baseplate is 5-1/2" in diameter with five mounting holes.  The overall height is 2-3/4".  There's a hex nut in the center on top that holds the assembly together.