I learned this trick from my friend Gale Perry, a very talented photographer and photo tour operator.
You buy a roll of silver colored mylar (you can find them on Amazon or in art supply stores) a make a cone which you attach to your lens with tape. It is a bit tricky to get it on without crumbling the paper, so be patient! The cone should look something like this:
Make sure you have plenty of light and that the cone is straight and watch the magic unfold!