[Small: for ukulele mandolin]
[Large: for guitar bass]
A.A.A. has released a dampening mat with 2 functions to reduce damage to the fret and to keep moisture conditioning inside the case. The name also says "Not only does it keep the inside of the case, but it also protects your frets from scratches.
Easy to use, just attach it to the guitar string and finger plate. Simply store it in the case to get the right amount of moisture inside the case and prevent scratches and dents from the frets and strings by moving around in the case during transport.
The moisturizing effect is the best choice for hard cases and semi-hard cases. Soft cases and gig bags may not have the actual effect