I've been down that road as well.
Although it would probably work fine with the bigger and heavier Pinewood Derby cars I couldn't get it to activate with the much smaller and lighter hot wheels car.
As I said I'm not using photo sensors. That is no longer an option although I appreciate your response. No matter what you do they are not reliable. To fully appreciate my situation picture spending hours organizing such an event. Everybody is there and excited to race only to have your finish line not work.