Simple DIY tiller arms .. why buy when u can build one...
One method i know is to use a 3 pin brass plug prong, use the hole with the screw in it to secure to the rudder post ( that hole is around 3mm i believe so u may need to drill it out to the right size if larger.) then just drill a few holes down its length for the linkage attachment point, you can tap out the holes if needs be to fit ball joints.
Or the other method i use is to bend a strip of 6mm brass around a nut and screw.. to form a U shape, then drill near the edge of the bend the right size hole to fit the rudder shaft. then using a hacksaw cut a slot into the hole you just drilled and widen it slightly with a file then drill a section of holes down the length again, to do it up tight, just slot a nut and bolt through the form/folded edge and tighten up around the shaft which sits in the hole with the slot. simple, old school, much stronger grip on shafts and no crap grub screws you will no doubt thread later.