With much appreciated help from Johnny and Paul. Over the less hot Tasmanian winter I decided a windsurfing trailer was in order.
Time over again I would start from scratch to avoid the massive calculations necessary to put a square box on what seemed at times to be a round trailer. Spent the first few weeks replacing running gear and re enforcing then out and up ;)
Result holds 15 sails, 10 masts, 7 boards inside, 6 boards outside, 10 booms, bags of gear. Also included a deep cycle battery to run LED lighting and allow for all important GPS charging.
Hidden a bed up the front because a "camper trailer" is cheaper on the boat.