Car booster seat to use as a spare for child's friends

[CoffeeTaster]

Hi,
I need to buy a booster car seat to have for when we have extra kids in the car. Aged about 5.
We have a "solid" one for DC but I don't think another one the same would fit between the two existing car seats. Also, it's to use v infrequently so I don't want one that's big to store. We wouldn't keep it in the car as we are usually bringing the dog and the buggy in the boot so space is at a premium. Nowhere in the house to store a big seat except literally sitting out in the hall or bedroom floor.
So is a "cushion" style one ok? Without the back?

Whatever you're prepared to put your own child on, presuming you have checked they fit legally (over a certain weight and height). It's much harder to guess the weight and height of someone else's child so if in doubt, I put them in the high back and my own child on the booster. Plus I'm really uncomfortable putting someone else's child on a riskier seat than I use for my own.

Just borrow the seat that their parents use?

I borrow the child’s own seat - I wouldn’t be buying a seat for very infrequent use by someone else’s kid, especially if I had no storage space.

Honestly, no I wouldn’t have a five year old in less than a high back booster either.

We always borrowed the seat from the parents, then it is their choice what seat they are happy for their child to be in.

Yes, really I wouldn't be comfortable with a child in a "less safe" seat than my own child.
Re borrowing a seat from the playdate child's parents - it might be the case that someone at short notice asks if I can pick their child up from school too and I wouldn't see them first.
So I will investigate seats with the back and sides too and just see what I can do with storing it out if sight

I have since realised there's a whole car seat topic I should have posted in!

Thank you all

Have you looked at a bubblebum?

@JustDrama no, but I'll Google tomorrow, thank you