AMuzi wrote, on Fri, 12 Sep 2014 07:59:42 -0500:

http://ezpourspout.com/

I apologize for not making my key point clear.

To my knowledge, NOBODY sells just the bung cap, at least,
not for the purpose of sealing a portable gasoline container.

The advantage of a bung cap (for lack of a better name), is
that it would seal as well as the god-awful contraptions you
pointed to, but it would be nice and simple and not break
like this all the time:
https://c2.staticflickr.com/6/5559/1...47f6daf7_b.jpg

Someone suggested we kluge together a cap from separate
components, but, really, there's no reason the cap shouldn't
simply be a single molded piece of plastic, with maybe only
one second part, which would be the rubber gasket.

I'm looking for a cap ... sort of like this:
http://www.oatey.com/img/sku/093432.jpg

Only the best will be able to find it on the net because
the *threads* for a gas can are ... um ... what?