terry writes:

Good idea, you have got me thinking now, I could just use balloons,
which will weigh nothing and fit easily into my flight bag, then I can
fill them with water when I get to the airport.

Balloons are more likely to rupture. Flexible containers of some kind with
sturdier construction might work, and indeed they might be better than jugs
because they'd be less likely to roll around; you could just set them on the
floor and secure them slightly in some way and they'd be much more likely to
stay put.