We need to get a second car seat, but finding the solution seems impossible, so I'm delegating to the MN hive mind

DH picks DD up from preschool once, maybe twice a week. It's only about a three minute drive on very quiet roads. He drives one of those tiny smart car things.
We have a Joie Stages in our "big" car, and usually we switch it between cars when needed, but this isn't going to be possible for much longer, so we need to buy a second car seat that will live in the smart car.
The big problem... DH has to have a passenger in the car relatively often, and usually with no notice.
So, we need a car seat to go in a smart car, and can be removed and stowed away within the smart car when there is a passenger. Is there any car seat that would work for us? I'm guessing the only option is that it would go in the shoe box boot. Does such a thing exist?
DD is about 103 cm tall and weighs about 15.5 kg. It is only for very occasional use and for a very short and safe journey.