This timing system is very easy to build, reliable and, best of all, inexpensive. The hole thing was built from wood, about 1cm thick. The sizes depend on what track you have. Also you have to play a bit with the light source, thats why i use a dimmer (the white box in the above picture).
In this picture you can see the inner of the gantry with the reflector (just some alu foil) removed. The bulb is at 20W and 120V.
The holes for the photocells were drilled and then shaped and are about 7x7mm straight to the slot. If you have bigger cars you may can drill the holes beside the slot, i couldn't. Don't bother about the cars leaving the slot there, they don't.
This is whats under the track, just a small platine with the photocells connected to a printer plug, no other elements there, no special wiring. Easy, isn't it? For more info how to build your timing gantry go to the Yarker Memorial Speedway (the link is on my links page). They also have the software for it.