Here is my trick.

The narrow width of the paddle shaft should be slightly bigger than the ferrule. Whittle the shoulder on the shaft and the shaft to fit the ferrule with a little side wiggle and taper the shaft at the shoulder to roughly match the diameter of the ferrule. Loose fit all of the parts and align the paddle shafts with the shaft vertical. Then wrap masking tape around the bottom ferrule, half on the ferrule and half on the shaft. Carefully take the top half of the paddle off by separating the ferrule halves. The tape should hold the alignment of the ferrule half and seal the crack between the ferrule and the shaft. The shaft should be well recessed into the ferrule on either paddle half. Since the ferrule is currently pointing up, pour epoxy into the ferrule so it just covers the end cut of the shaft wood. Some of the epoxy will settle or soak into the wood so fill again after it sets so the end cut is totally covered and sealed. Now repeat for the second paddle half. You have to be careful with the wet epoxy so not to get any on the mating surfaces of the ferrule. Just to be safe, I squirt the epoxy down a tube when the machined ferrule surface is on the inside. When you have done both halves the wood will be sealed water tight, the shaft will be straight, the feather will be right and without any offending screw heads.