Are you sure you need a flash? I shot a police promotion ceremony in a court room / board room earlier this year. There wasn't much motion during the ceremony, just guys standing at attention and occasionally holding up their right hand from time to time, so a slower shutter speed would have worked fine. Also the generic board room environment/surroundings allowed me to crank up the ISO a bit without huge amounts of visible noise. I wasn't going for artistic portraits, just trying to capture the event. I used my 18-135, and DFA 100 on a separate body. The 18-135 was fast enough.
But if you do need flash, I would agree the black foamie thing and a hotshoe-mounted tilt/swivel flash will be a reliable "go-to" combination for this kind of event. Google Neil van Niekerk's site for tips. You should check the room before hand to confirm the ceiling is white or neutral in color, and not ridiculously high like you might find in a wedding chapel. A normal board room should fit this criteria, but check it out before hand.
There was a local press photographer there who was wielding a softbox on a large flash bracket. I'm guessing it probably worked okay for fill, but I didn't see his results.
I have played around using a 1/2 gallon plastic milk jug and it works much better than my old Gary Fong diffuser, because it has greater surface area. But it doesn't look too professional sticking up on your camera. And anyhow, in shooting situation you describe, I think you can get better results bouncing the flash off the ceiling, or maybe a nearby wall (but probably just the ceiling will be fine).
Make sure you gel your flash to match the color temperature of the indoor lighting.
Last edited by Tanzer; 12-02-2015 at 03:46 PM.