Barquentine - I saw a video recently where a chap had a unique idea but didn't want to patent it until the prototype was made and tested to ensure it worked - The way he got around your dilemma was to split the parts required into 4 orders and ask different manufacturers to make each order so no-one actually saw the complete design, nor could guess what it was for - When the parts arrived, he put them together and hey presto - he had a working prototype and no one had the full design on record.
Good luck with whatever it is :-)