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.
Step 1: Supplies
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.
Step 2: Affix
I put a ring of super glue along the edge then taped it tightly with good electrical tape. Using cheap stuff is not advised because it doesn't stick well.
Step 3: Subtract
I have a crappy soldering iron I use for cutting plastic but there are also exacto attachments for wood burners that work alright too. You just have to cut out the middle. The read element isn't a wide angle by any means so you don't have to worry about it getting in the way.
Step 4: Win
All set! Attach your filters or flashes and go at it. Even without a filter on the hood blocks it from getting bumped while you are out and about. Have fun!