Buddy, I know you're new here, but you asked here for a reason. You already know full well that if it were easy and possible, you would have had a response quickly. Instead, you have two people vouching for the same easy method. Creating a disk image that's bigger than the file you're looking for, and putting your file in that is very nicely integrated into the OS. It's simple, straightforward, and accomplishes all the goals you stated.
Since nobody here is vouching for any one method, you might try google for "macosx encryption".
http://www.google.com/search?q=macosx+encryption and sift through the dozens of ways to accomplish exactly what you state.
http://macgpg.sourceforge.net/ may or may not do exactly what you want, but I guarantee it's harder to use.