Here's a hack I used on my S2 cameras: Pentax diopters are cheap and easy to find. They are rectangular in shape, and will fit over the Nikon eyepiece with some clearance to spare. All that is needed is to apply double-sided tape to the rear, then use a razor blade trim the excess so that the tape is only adhering to the plastic. Press on, and you're good to go! There will be no loosening, but the diopter is easily removed if you wish. The downside is that they're not "authentic", and certainly don't have that vintage look, but they do let you focus without trashing your glasses.