Travel car seat/ booster for hire car for 2.5 and 4 year olds

[Fmm]

Do travel car seats exist? We are hiring a car in France and cannot take our usual huge bulky car seats on the plane. Is the only option to hire car seats with the hire car (expensive and often not big enough for bigger kids). Kids are aged 2.5 and 4 years. Thanks for any suggestions.

[snowboarder]

We have a trunki boostapak for our 5 year old and it works well. Not sure for the 2.5 yr old though - last year we just bought a couple of super cheap light car seats and used them as less expensive than hiring them - but a faff to carry them.