Was just about to give up for today, when I found this from a UK supplier;
Solvit Medium Tagalong Booster Seat from Easy Animal
At just on $50 delivered

, I was delighted. I got the medium as at 7½lbs and 11" from shoulders to start of her tail, she's quite small really and they recommend a size up to allow the dog to stretch out a bit.
I've also got a seat belt attachable harness lead, which she wears at the moment anyway.
I'll post a pic when the

car and seat arrive and
"Her Majesty"

trials them both...