You didn't say how old your cat was. Best time to train a cat to travel well, is to start when they're kittens and take them on short rides (to the store, to visit a friend, etc.), then they don't always associate travel with a visit to the vet and needles. Unfortunately, you've done most of the things already I would have suggested, such as covering the carrier, tho I not a fan of using medication to calm a cat for travel. If the cat loves catnip, you could rub a lot of catnip into a few toys for the carrier, and also if you have fresh catnip give her some to eat. Some cats tho just start howling as soon as they get in the car, and unfortunately not much you can do about it.