I don't quite follow what you mean with the fill hole seeping, is that plugged when you aren't filling? What you are getting is likely a combination of condensation and splatter and a bit of gunk coming off the wick to make sure it gets a slightly darker coloring. Being as that surface is cooler than the wick and coil or even the metal below it, that is where it will go and you can't help it. As it builds up it will run down, and in your pic it clings to where the SS and the clear tubing meet. If you can (much easier with a lathe) you can make the floor of the chamber concave or add a dam so that this runs back to the mesh and can recycled and vaped. In the G-tank the chamber always comes out clean as a whistle, but the tank is covered in goo.