So I have a solution.
But I don't know how to make it or where to buy it. The local box stores don't seem to carry anything like this.
The threaded fittings that go through the barrel, and screw together sandwiching the blue plastic barrel in the middle need a gasket of sorts to form the seal.
Where do I find gaskets like this, or something similar to "form a gasket" material like I used to have as a kid for building engines? Came in sheets and you could cut out what you needed.