The picture attached is a cement vent pipe I found on the roof for water heater venting. Looks like the cap has been broken off, but I have no idea what was the original looks like. The diameter of the cement pipe is 4.75". The closest Lowes vent cap I found is either a 4in or a 6in Selkirk cap. But Im not sure if i get the 6 inch, are the screws going to penetrate the cement enough to get a good grip? I dont want it to be blown off in the future. What is the proper way to do this job?