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, Fenn Bluefin, Nelo 600, 550, 520 & 510, 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, Carbonology Sport Boost X LV, Fenn XT double.