If you would be interested to build a trailerboat, I'd be happy to work out the dimensions - I even think I could use a boatbuilding program to draw out the shape of the individual pieces of ply.
However you should be aware that something like a Trailerboat is quite difficult to build. The boat itself is fairly easy as is the overall trailer body, but the doors are a whole different matter - they're curved and don't have a square edge anywhere. Eeek! An easier method might be to change the trailer a rear-entry design like GeorgeT's Compact GT.
Andrew