"The Complete Illustrated Breeders Guide to Marine Aquarium Fishes" by Matthew L. Wittenrich, Page 222
The picture on the right shows a home made urchin of wooden skewers and what looks like a ball of epoxy putty. Marine grade epoxy putty would work well for this application! It's heavy like a rock, solid, and able to be punctured by skewers while it is hardening by chemical reactions.
Perfect.
Banggai cardinals aren't that difficult to breed. Read a great book, get the advice from people who have done it before, like Bang Guy. And have fun with it!