2 ideas
1 turn the plate so the hole is at the top, I doubt the pluming will allow this.

2 Use a hot glue gun and while the glue is still wet put a small piece of plastic bag ( leave the plastic on the glue ) on the glue and press the glue in to crack. I would have glue about 1 inch around the crack with plastic on it. You can use a heat gun to increase the work time by heat the holding plate before applying the glue.

3 J B weld? (epoxy )

4 Cover whole thing with glass cloth and epoxy?

well I guess that more then 2 idea. I would probably try 2 or 3.