Obvious Liquids' 140ml non-nicotinized neutral base "Fill'UP" bottle.
It is possible to make the DIY directly in the bottle thanks to the size of the container which is larger than its filling. It's easy, it's Obvious.
Here, the bottle is filled with 140ml of base but can contain 250ml of liquid. So, it is sufficient to add its boosters and/or its flavour concentrate directly in the bottle.