if your looking for a trailier that doesnt need to be built then i would get a trailex. i just got one to go up to the NA's and i got there and back with no problems. if you have any sunfish dealers in your area you should have no problem finding one put together. i think it is build for the sunfish might be wrong but it doesnt come cheap (around 700-800). if your looking for someting cheaper you can by a utility trailer and build something on it to hold the fish. im not shure the length on those though.
For a few ideas on what kind of trailer to get, go to the National Sunfish Class page at the beginning of this forum for a few ideas. Look under "Tips and Tricks" (right hand column) and also look at the FAQ's near the Vanguard Emblem. In the left column "how do I transport sail on my trailer", there are a few options for trailers. A few are the same as the "Tips and Tricks" listing. It would appear that many of the Sunfish sailors who do trailer their fish, favor the deck down style of carry over the deck up style. It is belived that less damage is done to a sunfish (damage to the interior foam and hull surface) when carried bottoms up. Hopefully, other readers of this forum will chip in with what works for them.
having just gotten back from the NA's I can assure you that there is no end to the creative designs for trailers. head to a local regatta and look around. The schedule is on the top of this forum. The key is not to have too much spring weight so the boat doesn't bounce over every bump on the highway.