After nearing completion of the wing top skin riveting, there are a few that I just can't quite reach. It turns out that back-riveting is actually very easy if you combine a back-riveting bar with a large rare-earth magnet covered in tape. In my case, this is a stack of magnets extracted from old hard drives.
Place the magnet above the rivet and then slide it sideways until it snaps to the bucking bar on the opposite side. Position above, and as close as possible to the rivet head. By placing it above the rivet on a vertical surface we're letting gravity assist in pressing the bucking bar against the rivet head and dimple area.