Riveted the internal ribs on the bottom skin and up to the preformed spar rib. Also riveted formed spar to the internal ribs of the left Flap. Installed the top skin as per plans and removed the rear spar for access. Was instructed to only cleco the end ribs on at this time. I used the leading edge jig that I used for the fuel tanks to hold the Flaps while riveting. The plans state to build a cradle like the leading edge tank jig, however, I found that the one for the tanks worked just fine. I then riveted the top skin on to the interior ribs.