It should be well ventilated, and be large enough for the pet to stand, turn around, and lie down. Line the bottom with a blanket to absorb urine in case of accident. Try not to travel in extreme weather conditions. If you must travel in hot weather, do so in early morning, or early evening. In hot weather, heat stroke can occur in minutes.
Animals can freeze in cold temperatures.
Also, it takes practically no time for someone to break into the car and take him. DO NOT allow your dog to run loose at rest areas. No matter how well trained an animal is, remember you are in a new place, and there are a lot of unusual noises. An accident can occur in seconds. If you are going to be staying in a hotel, call ahead and make sure your pet is welcome.