I didn’t have any helpers like the manual warns for bending and drilling the top skins for the flaps since they are 10 feet long. What I did was to use painter’s tape and “walk” the skin down slowly by reducing the length of gap at each tape location until they met.
Then as you can see in photo below, it raised the small axial length of bottom skin behind the aft spar upwards but didn’t do any damage. I used this to my advantage next. (Note: I reversed the aft spar to rib clecos so they didn’t interfere with the skin profile.)
I applied a little downward force to that uplifted skin at each rib location and it drew in/down the skin at the forward spar and ribs. I held it with one hand and drilled with the other—worked excellently. Both trailing edges were parallel. I drilled all the holes and everything was intimate… very pleased with that part.
Then, taking Dave Ammenti’s and Tom DeMarino’s suggestions for the trailing edge straightness issue, I went to Home Depot and got two 1-inch square 8-foot aluminum bars ($32 for both) and laid them on the store floor to check—they were extremely straight. I clamped them to the trailing edges.
I drilled through both skins and the tapered stiffener right into the 1 inch square aluminum and cleco’d right to the bar. Yes it took a little longer since some of my drilling was into non aircraft aluminum but it seemed a small price to pay for straightness. My buddy later commented I was being too picky and that it wouldn’t make 1 mph difference but I disagree. I’m betting I’ll need all the straightness on every part I can get. Especially 20 feet of wing trailing edges!