One way to hold molds together during firing is to use stainless steel bands– the kind that is used for holding hoses onto washing machines. They come in different lengths and will hold a ceramic mold together or a “plaster mold.”
But like the other person said, plaster molds are a one-shot mold for glass. Plaster molds can be “glued together” for the one-time firing by mixing a patch of wet plaster and properly drying the mold. I usually put a stainless steel band on that kind of mold if I think it might need more support.