There's not much question in my mind that the rudder will oil-can somewhat no matter what you do. One of the weak links (several threads in the archives mention this) is the lack of any connection between the stiffeners and the spar. Apparently in the older instructions, they mention to put some proseal between the stiffeners and the skin when final riveting. This apparently helps mitigate the oil-canning issue somewhat, but more importantly, helps keep the skin from cracking around the forwardmost or aftmost rivet. There are a lot of RVs out there (and I think the 9 rudder is probably worst) that end up with small cracks generated around the rivets because of the oil-canning when the stiffeners are not attached to the spar. I ended up with several of these small cracks on my tail (140 hours). A number of folks I have seen have just stop-drilled these cracks and report no additional growth. I have done this and put click-bond patches over the rivet head and crack, and will see what happens over the coming months.
So, my advice would be to either do the proseal trick or figure out a way to tie the stiffeners to the spar. If my cracks continue to grow, I will probably just build another rudder and do both of these modifications.
greg