I don't know of such a device, but I have a suggestion you could try. Plan a special outing, just the two of you, to go do something she likes. If she gets out of her car seat on the way there, stop the car, get her back in her seat, and give her a warning like, "You must stay in your car seat or we can't go to the ___." If she gets out a second time, stop the car, get her back in her seat and tell her you're going home now because that's what she chose. Don't give her another chance this time, but be sympathetic with her. Explain that you'll be happy to take her there next time, if she stays in her car seat. If she doesn't get out of her car seat on the way to the special place, be very generous with the praise. Let her know that staying in her car seat is a "big girl" thing to do. Come up with a chart, and some system of rewards for her staying in her car seat.