Stop drill at each end of the crack and keep an eye on it.
This should stop their migration. Once you are sure they are stabilized, you can fabricate a simple .016 patch, the size of a quarter or less, and glue them over the area with thinned proseal and paint. You won?t notice them.
I had one crack in my first 20 hours, then went 800 before my second one showed up, exactly like yours in exactly the same place. I pulled the patch on the one that occurred twelve years ago and it hadn?t changed. I am monitoring the second after stop drilling and may or may not patch.
The issue was well known when I built my rudder. I followed all the protocols in fabrication, trailing edge bend to reduce preload, RTV to tie stiffeners, etc...
If I get anymore, I will have to remain or rebuild. If I do, I will use .020.
However, as Mel says, lots of .016 rudders with no cracks on airplanes with high hours.