Help! Temporary repair to grey water tank.

I would use plumbers epoxy. It comes as a small round putty like stick. You cut off what you want to use. Then, you work it with your fingers till it is a uniform color. Put that into the crack with overlap. It will harden and may be a permanent repair. I have repaired buckets, plastic fenders, potty water tanks, plastic rings that the toilet hold down bolts fasten too and lots of stuff. I have never had a problem with it once it hardens. You can buy it at home depot and lowes. For your appilication it is better than eternabond.