I found 2 large dog cages (collapsible for easy storage) on Craigslist for $50. My plan this year is with 30 chickens to build some kind of shelf that will make the crates 2 story and fit the chickens in fairly tight so they can't slide around.
They will be processed late may/early june so it could be hot. I too have a truck cap (topper) and was planning on putting that on for delivery to the processor (1hr drive) with the side windows open. After reading this I'm wondering if it would be better to keep the cap off for ventilation. Would highway speeds be too windy for them in an open truck bed??