The end rib
Being its really the end rib of the wing section carry-through, its not technically the longeron. The cut-out will be in the sideskin, bottom skin and end rib to be big enough to allow unrestricted travel. Its near impossible to layout and make a cut in one shot. Start small and get the flap rod hooked to the flap and actuator then move it, mark it, cut/trim it.. rinse and repeat until all is good. Start with one side, then make a paper rubbing/template to lay-out on the opposite side. it will go much quicker! Of course, there is the hole in the floor additionally that needs done in concert. I made interior close-outs that cover that area and the flap actuator to keep PAX junk from getting lost or jamming things, so I get no breeze or noise from the openings.