I have built one.  Actually the trailer is basically a standard utility trailer with removable sides using stake pockets.  I built 2 wooden "A frames" that used the side stake pockets to help keep them in place and ran a gusset off of each "A frame" towards the trailers center to stiffen it up.  I built it for 3 canoes but you could easily make it for 4 or more.  The trailer can still be used as a flat bed, trailer with sides, or of course, hauling some canoes.  The change over is very easy and I don't have to worry about having 3 trailers.