For those who've never used an oscilloscope - the dim part of the scope trace is the afterglow "persistence" of the phosphor in the tube, from where the travelling light spot was
before the shutter opened. Neat!
For holding the light sensor (photodiode?) I have some adhesive magnetic tape, less than a millimeter thick. (http://www.maplin.co.uk
) A disc on the camera with a hole in the middle would perhaps do the job, holding a similar magnet surrounding the diode. It probably wouldn't impede the camera for other uses and removes the need for yet another holding device?
The little noise is not for putting the light on, then, so there may still be some mileage in seeing if its timing is more closely coincident with the shutter opening.