What can I use in my middle seat?

[Asleeponasunbeam]

It's an Audi A6 but seems really narrow across the back. My mum has had to sit in between DC sometimes and it's a real squeeze, despite her being a slim size 10.

I've got visitors coming with a four year old and will need to drive. Adult is too big for the middle space. I'm hoping I can fit something suitable in between the narrowest seats I've got (2 & 5 yo in car seats). 5 yo can use a booster I think? Any ideas?

If you can't fit anything it is legal for the child in the middle to go without. Not ideal but a possibility. I had to do it when my dd was still in a booster seat.

I don't like the idea but will only be making short journeys on local roads. I'm sure I can fit a backless booster in. Is that better than nothing? I assume so.