In an absolute pinch you can also do cardboard boxes with lots of ventilation holes cut in, but I have managed to collect a good number of cat sized crates. I also have three commercial chicken transport crates for taking my birds to the processing plant. They are designed to prevent them from standing up fully to prevent them moving around or pecking each other/fighting. It is counterintuitively best practice to pack them in tightly so there’s not a lot of room for movement. This is the chicken equivalent of wearing a seatbelt. Legs and wings are fragile, sudden movements or rough country roads, you want to prevent injury. Of course this isn’t possible with tiny chicks. We use this in transporting the sheep as well, 15 adult sheep will fit in a standard cargo van.
It seems a little cruel, but it is best to transport without feed and water if at all possible. It took me a while to wrap my head around this, but unless it’s going to be a very long journey having feed and water is actually more detrimental, it spills soaking them, they could choke on it, and it will most certainly become fouled with poop. If it’s going to be a long trip or have very hot weather, then a stop for feed and water out of the crate is how I manage it. I had an all day early fall trip where I decided not to process one of my roosters, so while the other boys were all joining freezer camp and I was working at the plant with DH, he relaxed loose in the van with some feed and water in the shade. Of course this also meant I had to catch him and put him back into a crate for the trip home. He didn’t really eat or drink much anyway.