I checked the tank seal on the rudder tailing edge by removong a couple of clecos - not set. I hope it does actually set up.
In the meantime, I made a cut-out in a 2x4ft piece of 3/4" MDF to accept my back-riveting plate. The cut-out holds the plate upper surface just slightly above the MDF surface - which should let me backrivet the rudder trailing edge without stressing the rudder.