Well I figured out how to get that third hand...notice the QuickGrip clamp that's holding down the trailing edge.

I drilled a hole through the workbench, removed the bar of the clamp from the handle, slid it down through the hole, and reassembled the clamp. This made it really easy to hold the flap down...

...and get a nice straight shot on the top pop rivets.

Once those top pop rivets were in, I finished up assembling the flaps except for the bottom rows of rivets along the flap spars where the hinges attach. I need to get the ailerons mounted before drilling the flap hinge so that I'm certain the trailing edges of the flaps will align with the trailing edges of the ailerons. I also need to ensure good edge distance for the hinge holes.

Back when I was installing the inboard ribs, I noticed these marks on the bottom skin. They occurred while driving the rivets that attach the bottom skin to the inboard rib.

It's because the flap bracket is right against the skin. If I had to do it over again I'd file down the edge of this bracket so there is 1/32 gap between the skin and the bracket.

Here's a pair of (nearly) complete flaps ready to be matched up to the wings.