Instead of using the rudder horns coming with the kit, I have chosen to use a rudder GO MODEL set.
That's a nice set, you can secure with a small flat key
I drilled a 3mm hole in a 8 mm diameter hard wood dowel.
Then drilling the rudder with a 8mm bit need some attention. So I started with a 3mm hole, to be sure everything is square, went to 6 mm then 8 mm.
Before gluing the dowel with epoxy, I used a large washer to sand the dowel parallel and flush to the surface, without damaging the covering.