Orthopedic Dog Car Seat for Small Breeds - Original Calming Burrow Nest for Car Anxiety Relief - 6” Memory Foam Booster & Travel Bed - for Dogs Under 25lbs, Frenchie, Chihuahua

I was in the same spot as you. I've got a velcro dog and he loves this one from Amazon It's basically his cozy spot in the car. Warm in the winter, cool in the summer ( has an additional summer water resistant cover for the pillow). Many straps and also attaching to the seatbelt. We have him in the back and he just snoozes as soon as he gets in. The only thing that worked for us. Tip for transition: have it at home for a few days as a bed, it's also orthopedic. Then take it in the car and let your dog just rest in there for 10 minutes, go nowhere. Do that maybe 2, 3 days. Then take it for a small ride. Go slow, your pup will adapt. If very stressed, go back one step and repeat until comfortable.