Beneath that wing nut is a rubber gasket. Make sure that is in good condition as well as replacing the cowl seal. I used garage door seal I saw on a thread on here. Make sure you silicone or rtv behind the new seal.
Its also a good idea while you have it off to clean the cowl area and put some wire mesh over the holes to keep leaves out and when you reinstall the cowl, slap some silicone on the inside of the steel ledge where it meets the firewall. This will keep the rest out.