A cardboard box with lots of shavings is ideal. This way they won't slide around when you but on the brakes. If you make it dark, they will sleep. They are a heck of lot easier to transport than a cat!
I use a dog crate with shavings, they just sit down and ride calmly, I had the radio on and certain songs and tones got a soft cluck from the 3 in the crate lol. they come right out looking for food. when I went to the poultry show, I saw that most hauled theirs in crates also, or boxes made for poultry.
I have found that they are a lot more calm when it is dark. So, I use a cardboard box with a few holes. I always make sure that they have a little bit of room to move, but not enough to make them bang up against the side when you turn corners, etc. You can put more than one to a box but it (obviously) needs to be bigger with a few more holes and room for all to wiggle a bit.
I don't think you will need to worry about it being too hot in the box at this time of year! (LOL)
I use cat carriers. I took 10 roos to the processing facility that way. I was ready to kill them myself after listening them crow in the back of my SUV for 2 hrs.
Everyone else that dropped their birds off had old chicken crates, cardboard boxes, etc. My boys arrived in style, at least.
Any type of carrier will do if not them a box for that short of trip.. If I take one or two of mine somewhere they usually like to sit on my lap and look out the window. Never had a bird freak out on me while driving.
This past week, i transported 2 of my boys on a car ride (to a butcher). It was an hour and 20 mins away but i loved my guys and wanted them to be comfortable. I carried them in a pet carrier in the passenger front seat, air on them and even stopped at a fast food place and shared my burger with them about an hour in (last meal). They seemed to like the noise of the radio on; clucking with certain songs.