I have just tried out a cable release and I am going to make a metal clip to hold it just below the top of the side handle at about 45 degrees. I am used to using a left hand shutter release as I use my RB67 with the left hand grip.
A flash would look cool. I was thinking about building something which looks of the same era but with the flash tube and circuit from a more modern flash. I could hide the batteries and the PCB in the spaces previously occupied by the shutter curtain rollers.