Reversal rings are great fun for cameras. They allow you to get super close using your existing lenses in a new way for very little cost. However when you're doing so you are exposing a delicate element of your lens which wasn't made to be exposed. This little trick will not only allow you to protect your rear element but it will also let you attach other filters and flashes.
Most photographers have an extra back cap laying around or if not they're cheap to pick up. Most photographers also have extra filters or old ones laying around as well. Mine was a little worse for the wear so I removed the glass from it. I would recommend going with a fairly standard filter size like 58mm but if you have others you could use them as well. I didn't take a picture of my super glue but I did use a little bit of that to sturdy my union a bit.