>Only downside I can see is if he starts to chew bits of the car he can access
Good point. I have puppies in a smaller rectangular crate to start with but I also don't really have a 'chew' breed!!
Another alternative is a custom fitted crate which is exactly sized for your boot. Dog Pod from Hamster Baskets also feature pull out extensions which can be used if you put one of the seats down. Most of the aforementioned companies will make you a crate and some may already have your cars dimensions on record.
Finally, don't forget that if you go to somewhere like Pets at Home they are pretty good at letting you try the standard sloped car crates for size, both in the car and with your dog...