The medium Snoozer is 22 inches wide and recommended for dog(s) up to 25 pounds. Should be a good fit. It comes with only one tether, but you can buy a second tether individually. The tethers attach to your car seat belt.
We have the large Snoozer, which has a seating area of 25 inches wide. It gives my big 9+ pounds boys tons of room.
__________________ Kristin, Max and Teddy
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