Is to use 2 stop watches. The first on is clicked when the plane passes it, and left running. The second one is clicked when the plane passes it and left running. Then both watches are brought to one person who stops them both. Time of the second watch is subtracted from the time of the first watch. That is the time thru the trap.

No flashlights, signals, nothing. And having one person stop both watches is pretty close to instantaneous.

And I'm not out to set a new FAI 10cc record, so I can use a course longer than 358 ft or whatever...the longer time makes the effect of clock error much lower.