I have built quite a few trailers over the years, a couple of cat trailers (the easiest of all to build) and a couple of standard mono trailers, it is quite easy, but very time consuming when you are doing stuff in your home garage, you can buy almsot every part for a trailer off the shelf these days, making it much easier to do a quick job on the small fiddly bits and pieces, if I was building a mono trailer, I would make it in a V to suit the boat and just have a cradle with skids on it, that way the boat has to go on straight and centred everytime, but the trailer is more or less only good for one boat.