Different types of direct vent caps 

I have a unit like the one to your right (the viewers left) that has been damaged by hail. I am looking to replace this. Is it a fairly simple DIY if it is simply just that cap that we would be replacing?

Do you put caulking on all 4 sides? It seems as if my fire place is fogging up after i put caulk on the bottom section of it since it was not caulked

How about snorkel? Do you install any ?