Caps for Nikon Polarizers
Nikon makes great polarizers, whether you shoot Nikon lenses or not. They do, however, have a unique design feature wherein the front element diameter is larger than the rear in order to minimize the chances of vignetting. The only downside to this is that it can be hard to find a lens cap to fit the larger front element. Here's what I know about what works:
The 52mm polarizer is easy since a standard Nikon 58mm lens cap should fit it just fine. Similarly, the standard Nikon 72mm lens cap fits the 62mm polarizer quite well.
The 72mm and 77mm Nikon polarizers though are a bit more of a problem. No standard Nikon cap will fit them so we have to look elsewhere. Some people have suggested that the solution is to take your polarizer to the grocery store and try on every butter tub lid and Pringles potato chip cap you can find until one of them fits. While this might sound tempting, most plastic caps like this outgass and will cause a film to form on the front of your polarizer. Not good. For the same reason, don't get tempted into using the cap from a tennis ball can on your 62mm polarizer, although it probably does fit.
I can only speak directly to the 77mm polarizer since I don't have the 72mm, but I understand that both have the same front thread size. After much searching, I finally found that the Contax K-84 metal lens cap (89mm diameter) slips over the front of the 77mm polarizer perfectly. Fits like it was made for it. You should be able to find this at larger camera dealers such as B&H, but as another option, there is am 86mm Sigma lens cap that is reported to fit as well.
Why Nikon doesn't make caps to fit all sizes of their polarizers (or at least let you know when they do) is a bit of a mystery, but there are solutions out there. Hopefully this tip will save you some time on your search for one that will work for you.