Occasional booster seat for a 5 year old?

[GnarlyOldGoatDude]

Have 3 DC of my own, all in "proper" seats across my middle row of seats. I've got a 7 seater with 2 seats in the boot that are usually folded down. If we occasionally wanted to give a lift to another child, are the sit-on booster cushions a complete fail, safety-wise? I know high back boosters are much better, but what's the best option for a very occasional or unexpected/ emergency lift for another child?

Any advice would me much appreciated! I don't want to spend a fortune as it wouldn't be used often, and not too big as the plan would be to keep in the boot in case it was needed.

How about a Bubble Bum - inflatable booster. I use one for my 6 year old and it's fine.

Why not just get a standard booster cushion?

Thanks both- yes I'd been looking at the bubble bum, but mixed reviews about it deflating and slipping.

I was posting as I'm sure I heard quite recently that the basic booster seats (as opposed to a high back booster) weren't considered safe? Will have to google a bit more

I'm the same, we have a "5+2" car and I always have a backless booster cushion in the boot 'just in case'

High backed boosters are a lot safer, especially for 4-8 year olds, but if it's a case of using a backless booster or no child seat at all then a backless booster is fine.

For occasional unplanned use a backless booster is better than nothing, and obviously you'd use it for the oldest/tallest child that you were putting in the car.

If you have space to store an extra HBB in your car then do that.