I also came up against this problem when building my 6 meter catamaran here in New Zealand. It seems that these days the only off the shelf rudder fittings available are for dinghy size yachts. I did not want to use stainless steel fittings because of weight and complexity in having them fabricated. I thought about the dyneema lashing system and that certainly has some big plusses in simplicity, weight, and low cost. In the end, and being a person who likes to experiment, I fabricated my own fittings from carbon fiber. I already had carbon cloth from other parts of the construction, and after only a very small amount of trial and error I have ended up with some successful rudder fitting which cost almost nothing, only a small amount of carbon cloth and epoxy resin, and some stainless bolts and nuts. I made the spacing washers from some super slippery polysomething plastic I had lying around from another project.
Oddly enough these are almost as per the suggested sketch Arne posted. On the rudder box a flange is constructed of several layers of plywood which basically encases the box top and bottom. Around the outside of these flanges are a number of layers of unidirectional carbon fiber. The bolt passes through this flange in a 10mm carbon tube sleeve which acts as a bearing. The transom fittings are just the simple carbon brackets as per the photo. These brackets were made up over a male mould. I also later fabricated the boom gooseneck from carbon fiber as once again it is impossible to purchase an off the shelf fitting which is suitable for the size of boat, although a junk rig yacht will not have that problem!