Wings: Flaps. Back riveting top skins.

The two aft-most main rib rivets on the top skins of the flap are back riveted to preserve the trailing edge shape.  I bought a 12"x12"x0.25" piece of steel ($30!) and some 6"x2"x0.25" poplar scant spacers to create a reasonable surface to work against.  I even used my other back riveting plate to help out.

Placing the flap on this surface and weighting it down to keep it from falling over (since it's nose-heavy with all the fore clecos), back riveting was trivial.  

Here are a couple of shop heads.

And the left flap's top skin completely in place, waiting for the wedge to get attached to it after the bottom skin.