I have tried silicone and just a tight fitting tube. Both have failed for me. What I have done now (and has worked consistently for the ones I made for friends) is:
Use a Gatorade bottle. I found that soda bottles tend to leak through the cap sometimes. It caused me hours of frustration until I used a Gatorade bottle (I use the bottles that are close to 2 liters and also the one that's a gallon or something). Seals perfect every time.
Drill the hole in the cap, put the tube through, then wrap some duct tape around the cap. Make sure there is a decent amount of tape above the cap. Fill it with Gorilla Glue (doesn't have to be to the top, but thick enough). Let it dry over night.
This will definitely work. I have never had a problem with this method.