At my old job there was a black BEST Access Systems-branded box mounted to the wall of the server room that had circutry for an access control system that was not being used anymore. I have a bit of a hobby dealing with BEST locks, so I decided to turn it into lockable storage for the keys I have lying around.
My friend cut down and painted a piece of wood to use as a backer, we drilled holes to allow access to the original mounting holes on the back of the box and installed small brass hooks in a grid of 6×4. We used industrial adhesive to attach the wood board to the back of the box, and I modified an Illinois DUO cam lock to replace the lock that was pre-installed (I didn’t have the keys for it anyways).