Hi Kiwi Dave. Don Kiesling; surfski(at)gmail(dot)com makes very good after market rudders. I've just got home from testing a new DK downwind rudder on my XT. To say I am pleased with the performance would be an understatement. He can make them to fit most brands of ski and can do pretty much any length, chord, leading / trailing edge angle etc that you want. He posts them world wide. I have an 8" low angle DK rudder on my Bluefin which has plenty of bite and has never caught a weed despite me deliberately paddling straight over all sorts of seaweed. I paid in $Aus so they were not cheap (and even dearer for you I suppose). However, the new rudder has transformed the handling of my XT which means that I am happy to keep the old XT instead of upgrading to a new boat which will save me thousands of dollars.
Current boats
Epic V10L Ultra, Epic V9 Ultra, Carbonology Sport Boost X LV, Fenn Bluefin, Nelo 510, Fenn XT double, Nelo 600, Expedition Kayaks Azure, Mirage 732.
Previous boats
Spirit PRS, Fenn Swordfish, Fenn XT, Fenn Swordfish S, Think Zen, Epic V10L Club, Carbonology Sport Boost LV