A baby's gender can be a hard thing to read by innocent passer-bys so a gender-appropriate car seat cover can help! But the problem is, car seat covers can run up to 60 bucks!
You can make your own with a lot less, by taking your current car seat cover, and tracing the pattern of it onto tracing paper that can be pinned into a new fabric. That way, you can make multiple covers for multiple babies. You can even make multi-layered ones that you can stuff with cotton to make them warmer, then sew them shut. Another option is to witch-stichery them together.
Or you can go to your local fabric store and pick up car seat cover patterns. They are seriously cheap, and you can pick up your fabric while you are there. Just make sure you pick machine-washable fabrics. Those will make your life ten times easier.
Another option is to buy patches that can either sew or iron on your current car seat cover in order to jazz them up and make them more gender-appropriate. But you might not want to change the ones your already have.