what to use for a clean out plug

Hey folks, thanks for the advice so far! Out test run outdoor was successful, starting the build indoors. We're laying out the exit flue and wondering, what to use for a clean out plug. We have an 8" exit flue and have two Tees for cleanouts (placed so we can access all parts of the whole run for cleaning). I have a metal CAP made for the Tee but it's not a tight fit. I'm worried about noxious gases leaking out. I also saw a metal PLUG is available for the 8" pipe, which is different than the end cap that I have, and might be a tighter fit, I don't know. What do people use to plug the clean out?? I saw a video clip with Ernie and Erica and they pointed out their clean out plug which they said was only $12 and seemed to fit tight. Anybody know what that item is exactly and where can we get one? Or have any other suggestions?