One obvious method would be to make the bottle in two halves and join them together with the contents in place. Either split it down the middle or -- this might

One obvious method would be to make the bottle in two halves and join them together with the contents in place. Either split it down the middle or -- this might be easier to do without it being noticeable and without damaging the contents when joining the pieces - make everything apart from the bottom of the bottle in one piece and attach that at the end. Whether that's actually what they do, I don't know.