To get a consistent radius around the tail feathers I came up with the idea to make a radiused sanding block myself. So I bought a router bit in the exact radius of the control surface edges (the ply, two layers of balsa and some extra space for the sanding paper), and routed a slot in a piece of soft wood:

Then, with the help of a long wooden dowel and Pritt stick I glued a strip of sandpaper in the slot:

And from there on, it really is a very simple process. The sanding block automatically centers over the edge because of the matching thickness. Pretty happy with the result!

Difficult to catch in a photo, looks even better in real life, but the radius is nice and consistent around the elevator and the thickness of the edge is perfect for imitating the original tube structure: