A plastic welder would plug that very easily. I cut the entire top off my grey water tank (Bus Depot), inverted the cut off section and plastic-welded it back on. They work well. 

Bob

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> Subject: What is this hole....?
> 
> I have the water tank out and I’m sure my water leak is from this hole: http://www.feddishbasses.com/vw/fringe.html 
> 
> Sorry for the grainy shot.  Anyway, what was here originally and what should I do with it?
> 
> Thanks,
> Bryan