I have a PVC furnace exhaust pipe coming out through my roof. Unfortunately, the guy who installed it used the wrong size flange. So instead of the rubber rising upwards against the pipe, it is sunken downwards, and water puddles in the depression and eventually drips inside. I tried using roof sealant caulk, but it didn't do the job. Is there some kind of thick, sticky sealant that I could drop in there to solve the leak?