Seat covers made from cloth could be the only alternative and I agree that they do tend to get dirty. However, you may wish to follow a simple trick that could keep temperatures inside your car somewhat down. Just roll down the glass windows of all doors of your vehicle when the car is parked leaving a gap of around 2 to 3 millimeters in each window. This would allow for cross currents of air to flow through your car, keeping down temperature by some degrees.