Get a large-dog kennel and outfit it with small litter box, water and dry food dishes, cardboard-box “house,” open on side, with comfy bed inside, large enough (or so) for all cats. Throw a large dark sheet or packing cloth over most of the kennel so they feel extra safe and cozy.
Line the bottom of kennel with those large-dog pee-pee pads for easy, daily clean up.
Hard to describe what I’m trying to say. Wish I could draw a picture for you.
The risk of accidentally losing a frighened, bolting cat out the car door is too great. The large, outfitted kennel is essential. Having their own secure place in your car will relieve stress. Yours and theirs!
Calming meds from the vet are a good idea, too, if needed.
This is the kind of crate we have used with great success (wire mesh not Molded plastic):
It comes in many sizes. We use the largest, 48”.