To fasten a cuttle bone to a bird cage, place the cuttle bone hard side down on a hard surface and insert a sharp object such as a scissor blade, ice pick or skewer through the cuttle bone soft side until you make a hole on the hard side. Then turn it over and enlarge the hole.
Then insert a plastic zip tie through the hole. These plastic ties are readily available at stores such as Lowe's.
Fasten the zip tie to the cage making sure the cuddle bone soft side is toward the bird and the hard side is toward the cage wall. Cut off the extra end of the plastic tie.
Place a cuttle bone in each hens cage.