As anyone here knows, you're first design may not be your best. I had a plan for my water tank placement, and then changed my mind for what I believe will be a better plan. Wait! I already drilled a hole in my poly tank and I need to move it. No problem, drill another hole. But what do I do with the hole I no longer need? Patching poly isn't easy. I don't believe there's a good, 100% no worry in the future glue to use. I'm thinking of some kind of expanding plug , just don't know where to look. Any ideas?