Thanks for the various replies.
I got hold of a rudder of which the blade was just slightly larger than specified on the plan, so I thought that would be a good start. However, I hadnt appreciated that the tube diameter could be an issue. It would appear that this one has a 5mm shaft, and the tube is about 7mm OD. Given that the keel is 8mm, this is not great, in fact it will just about cut the keel in half in a longditudinal sense. I realise that i could put doublers on, as with the prop shaft, but this is not ideal, I imagine.
Plus there is the point that there is a flange on the bottom of this black plastic tube, which is going to be proud of the bottom of the hull (a point ) , unless recessed into the hull (thereby weakening the hull further). It looks to me that basically this is tailored for a round-bilge / flat bottom, and probably for a thin skin, e.g. grp, but that's just a guess. So this has got me thinking about making a rudder. The plan appears to use a 4mm shaft and approx a 6mm OD tube, perhaps one might even get away with 3 & 5 ?
Anyway, I had a look on ebay etc for supplies, it seems possible to get hold of the materials and not at great expense (hurrah), so if I got some of that, and some brass sheet, the next thing is, the best way to attach the shaft to the blade. Would solder do it ? I have a vague idea that perhaps one might have to braze ? I have a soldering iron, and some flux, but brazing is I think beyond me. On the other hand, perhaps a right-angle bend in the bottom of the shaft to give an "L" shape(viewed from the port side), and then build a blade around that ? How much of a concern is water ingress up the tube (I appreciate that the higher the tube, the less that would be a problem, but am thinking of the "ram" effect of the water moving at high speed past the bottom of the rudder tube).
Thanks