I used a 24" crate for Juno from the day we brought her home at 10.5 weeks until she was 4.5 months old with no problems either in the car or in the house. I just used to carry it flat from one spot to another, oh and upstairs for the first couple of weeks for her to sleep in the bedroom
We had a 24" crate for Bailey, but having an even more impracticable car than you (Fiesta T2) the crate never fitted the car, we've used a harness and seat belt clip after the first trip home, although I sat in the back with Bailey for the first few trips. We used the crate at nights until he outgrew it and since then he's been good as gold and happy without a crate. When we go out he is either with us or (now at 5 months) he is given some treats to settle him and he is fine in the kitchen if we need to leave him home for a hour.
Well the crate is handy in your own home too, I use the crate for Millie to be able to settle down and nap when she gets overtired (can tell when she starts nibble a lot she's tired) and I don't want her to teeth on our furniture during the night so she sleeps in her crate. I never used the crate as a punishment, i want her to see it as her safe sleeping place. We used the pet carrier when we brought her home and after that harness/seatbelt for shorter rides and the crate for longer ones. But we got a jeep with a big trunk so we can fit the big crate. Don't forget to buy some chew toys for your new pup I bought nylabones I think it was called, Millie looooves those. Especially a blue smaller one that has knobbles on it (I guess it feels good when it itch in the mouth) Since Millie's stomach didn't like those puppy treats stores got I introduced apples and bananas second week we had her home (now she would do anything for a piece of fruit and she always try to steal my daughters banana lol).
We brought both of our pups home sitting in my wife's lap. With Tilly we stopped at a park on the way home, and with Cooper the drive with us was shorter and we just went home. We have a pet divider in the back of the Outback so when we want to, we can put the dogs in the wayback. If they aren't muddy, we usually just let them ride in the back seat of the Outback or AeroStar. If one of them rides in the MX5, I put a seat cover on. We never crate in the car, but none of our dogs have ever wandered around in the car. They either sit or more likely lie down and go to sleep. They prefer the back seat to the front.
After saying good buy to his little litter mates, his mum, hid dad, his Aunty any the owners and their parents we bundled our 8 week old pup into a cardboard box in the boot of our old VW golf and set off down the road. I got into the passenger seat as usual and hubby drove off. Within less than 5 minutes the cardboard box was upside brown with the puppy under it. So on we go. 5 minutes later the puppy was now on top of the box. 5 minutes later he managed to climb over the back seat. No crying, just being inquisitive, independant and determined. By this point we had just turned into the motorway so hubby had to find a safe place to pull over, I climbed over into the back seat and held the wriggly little bundle all the way home. Some dirt of harness and seat belt clip and to set next to him would be better than our inexperienced fun and games.
