Is there a pattern somewhere that is like a box that a ring or other jewelry would be gifted in? My first thought was simply to create a quilt with squares and pin the brooches to the squares but then got to wondering about a "box". As another commented, this could get pretty heavy depending on how many brooches you want to display. And black velvet or a blender that gives that effect would be perfect for showing off each one.
I am reminded of a craft project that my mother did many years ago that she made a shadow box using a large antique picture frame. She used jewelry to create a "Christmas" tree shape inside the shadow box. The weight of the jewelry was supported by the back of the shadow box.