Revering your lens in order to take inexpensive macro photos is a common trick but it exposes the inner elements of your lens in a way they weren’t intended to be. This simple filter adapter protects your lens during the reverse-lens process.
Instructables user Jeonc built a simple and cheap mount to attach to the end of his reversed lenses. The mount allows him to protect the end of the lens (both the glass and the contact points) as well as swap out filters for special effects with his macro photography.
Hit up the link below to see how he turned a back cap and some basic supplies most photographers will have laying around into a cheap and effective filter holder for his reversed lenses.
Reversed Lens Filter Adapter [Instructables]