I just use an old computer monitor box. The bottom is lined with a plastic potting soil bag with straw on top. I dump the straw and rinse off the bag so I can reuse it each time. On the sides of the box, I cut out view holes for them to stick their head through and look around. The box gets wedged on the back seat between the two front seats of the car, that way they can see us and look out the front windshield- and the box doesn't move around. They seem to like the option of nesting into the straw and napping or sightseeing and talking to us.
I also cut out a rectangle on one side, leaving the bottom attached. It acts as an door and exit ramp so they can walk in and out on their own.
I've thought about leashes, but our little ones are so independent, might be like most cats are on leash- no fun for anyone involved. I just know I couldn't keep them in a stroller (and them be happy about it anyway). I'm pretty good at herding them and they follow us around. But I worry about all the cats and loose dogs in our neighborhood so we only take them on car trips to isolated places. I'd love to take them for walks to the park so they got out more often.
Have you had encounters with predators? What did your duck do?