Not really a trivial task. A second false spar box (similar to that of the aileron) would be needed, and since the flap would probably have more travel than the aileron, it would probably have to be more beefy. Plus you'd need the linkages and controls to MOVE the flaps, and appropriate designing to minimize the potential for a split-flap condition.
If one were serious about minimizing the chord to support transportation through a restricted stairway, I just wouldn't glue the inner ribs in place. End with the rib on the outer side of the aileron notch. Fit-check the other ribs, but don't glue them together or to the wing. With the outer ribs in place, you can lay out and carve the wingtip bows.
Then haul the wing upstairs up to the garage, attach the rest of the ribs, and finish the aileron false spar.
Ron Wanttaja