When I said box, I meant a proper plastic cat carrier.
The RTC aspect had occurred to me as well, although no way is he 6kg. He was 2.5 kg at the vets a few weeks ago, and I'm hoping he's put some weight on since, but I doubt he's put 3.5 kg on !
I suspect, as you say, that the association
car == vet / cattery / badness
...has been made.
He doesn't particularly like the carrier, for the same obvious reasons, but it's also clearly the engine, since the moment the engine starts, he starts meowing in a very plaintive way, until it stops. You wouldn't think one small cat could meow continuously for a long(ish) journey, but he can. He's always done this, I think he made the association with his previous owner, of which we have no knowledge, he was taken to the Blue Cross as a stray.
I guess I'll stick with the carrier, he won't like it, and he's going to have a visit to the vets, and the cattery, within a couple of days of each.