I bought the online, eBay product. After I saw what I got I went to a local watch repair shop and got a crystal sized perfectly to the space. Longer lasting than plastic from eBay and takes a licking and keeps on ticking! (thank-you John Cameron Swayzie)
RTV is OK but I used JB Weld for peace of mind. There isn't any significant pressure on it. You might even be able to save the chrome bezel to reuse, or look for a grommet that may fit if you want a metal finish. Maybe one of those A-C red o-rings for a racy look.