Not sure if this really helps as it pertains to flush rivets. I found attaching the rudder stops with flush rivets much less of an ordeal than I thought it would be.
The aluminum angle used to make the stops is significantly more forgiving than the skins used on other parts of the fuse. It's pretty obvious that a mushroom set isn't going to get to the rivets, but I found a 5 1/2" straight rivet set made the job pretty easy with no modification needed.
Flush rivet set used https://www.yardstore.com/riveting/r...5-1-2-straight
I turned the air pressure up a bit (50 psi I think), took my time and ended up with the finished product below. I shot a little primer over the rudder stop to touch up a couple of spots.
By the way, I installed the rudder stops AFTER setting the last rivet that holds the angle stiffener in place. This rivet is located close to the lower left-hand corner of the rudder stop. As I recall, the manual has you set these rivets after the rudder stop is installed. I think that would be quite a challenge....at least for me.