I fluted the flanges to flatten the flimsy fragments, but finally folded.
Things don't stiffen up until the aft section is riveted together. Once it's all in assembly it gets pretty rigid. Monocoque construction for real!
Remember not to make the flange 90 degrees to the web, but something more than that to make them match the angle of the skins (since they're a sort of "cone" in shape).
Larry, I didn't have to flute any of those bulkhead parts at all. Maybe there's enough slight variation in these parts, that yours might require it. But I didn't have to touch mine. Cleco all the skins on and see how things fit, if you haven't done that already. You might be pleasantly surprised.