The easiest (but probably not the best) method would be to use the camera's contacts to operate a relay and use the relay's switch contacts to fire the flash. This will probably not introduce enough delay by itself but to increase the delay, I would use something like a 5 volt relay with a 9 volt battery and have a series resistor in line which is similar in value to the resistance of the relay's coil. I would then experiment with connecting capacitors across the coil to delay the turn on time.