David,
I envision the shell body and explosive cavity to be formed in the normal manner.
If you look inside the cage, you will notice that the ribs and slots are angled towards the middle of the cone that forms the center hole in the cage. These tapered ribs and slots are so angled for ease of removal.
I believe that a sand cone is placed in the center of the body and sand wedges are placed at equal intervals around the body which extend and touch the sand cone or core of the cage.
After pouring and cooling the body is knocked free of its sand box, the sand is then removed from the cage cone area and the sand wedges knocked loose and the sand removed from the explosive cavity.
There is probably a jib to permit equal spacing of the sand wedges. Remeber the sand corne and wedges are cast separately into a very hard texture.
Please understand that my drawing was a quicky and only meaat to convey the idea, not profice specifications.
An alternate method would be to make a wooden pattern of the cage core with wood wedges glued in positon around thid core or cone. Make a mold of the the completed core and wedges, make a hardened sand replica and place it in the body mould. Like Joe said the casting mould would be made in two halves.
Regards,
John