I used an old rad cap (new cap first point of call when I got the car) and removed the spring etc. I then used a hole borer on my drill to make a hole the size of the threaded part of the 2l lemonade bottle which is the "tank", the threaded part of the bottle was screwed through the hole in the cap, and then the bottle cap was refitted and a hole bored though it. The result is very secure.

