Finding car seats for a Renault Trafic van

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My husband has a Renault Trafic van and needs to drop our 3 year old and 7 month old off at nursery. We have a car seat for the 7 month old but I'm trying to find a car seat for our 3 year old. The van does not have isofix so it needs to be seat belt fitted. It also needs to be slim fit as there is only room for one thay is about 36/37cm wide. Does anybody have any suggestions?
Thanks.

That is tricky to do. Most car seats are around 42cm minimum. What car seat have you got for the 7mo? If this is bulky it might be that you can get 2 slimmer ones which would fit together more easily.