I don't have any folkboat specific experience with trailers but I have used a bunch and looked at building one for my 30'er.
If you haven't looked at their site, I would recommend taking a look at Triad trailers. The trailers are pretty expensive but in my opinion, they are the best out there and have some nice features for float on and off. They may have even built one for a folkboat before.
Getting the boat lined up into the trailer is usually not too hard if you have keel guides which are easily built and installed. The trick is getting the boat to consistently sit in the same place on the trailer fore and aft each time. Fin keel
boats are often run up until the keel hits the front of the keel guides but as the boat is hauled, it typically settles back on the trailer because the boat starts level and the trailer does not. With
full keel boats, there is typically a v block which goes against the bow. Again, the bow can pull back from this as the boat comes out of the water. The best option is to really fully define a point up at the bow almost like a pin which constrains the height and horizontal placement so that the keel will ground out in the same place each time. Otherwise, you will find that you need to slack off the
winch cable as the boat comes out of the water. I hope that this makes sense, please let me know if I should try to explain it better.