Here’s my prototype of a watertight connection. It’s built in a Chinese food container, just to see if it works. I tried it with only one rubber ring on the inside, but that failed (on the left: NOT watertight)

I used metal hollow thread pipes that i found in the store with the lighting products (!?). It came with the flat nuts. I used pieces of rubber from “Grolsch beugels“, people from the Netherlands will probably know these bottles of beer, NEVER throw away that little red ring! (-: Also on both sides a big flat metal ring to press the rubber ring to the container.

So, around the pipe, from left to right, there is:

• - flat nut
• - metal ring
• - rubber Grolsch beugel ring
• - container (through an 8 mm hole i drilled with a very small hand screwdriver)
• - rubber Grolsch beugel ring
• - metal ring
• - flat nut
• - transparent tube out

I needed to heat the tube (6mm inner, 9mm outer) to fit the pipe. I used nose pliers to widen the tube wile holding it over a candle.