I prefer screws, but in order to do screws correctly you will need to have a 1X2 framing between the shell and lining on the interior of the opening. The wood framing is cut and glued underneath the shell then the vent screws are screwed through the shell into the wood framing. I only use stainless steel screws. I use butyl tape over the head of the screws or rivets, much easier to clean off later if one has to be replaced. If you are going to caulk never use silicone, use 3m marine caulk.