I have a goodly assortment of bottles that might work, if I want to use a small, airline-type bottle.
Most are glass, like the Sabra Liqueur bottle which is a lovely shape but it is glass. And who knows how Spain is feeling about Israel these days so best to err on the side of caution.
The Drambuie bottle might be the best as it is plastic so light and easy to mail. The magenta bottle that is shaped like an old medicine bottle was made in Taiwan but has most interesting
writing on it. It says, Nectar of the Golden Life of Health and Vitality.
By far the most interesting bottle that would be marvellous to send is a fake Champagne bottle that has a hidden compartment. Not that hidden, mind you, as the plastic "cork" is absolutely noticeable the moment one turns the bottle upside down. There is a large cavity, the entire size of the bottle, and it easily holds a standard typewriter-sized piece of pager when folded or rolled.
Alas, it is very heavy and totally breakable.