In researching weather balloons, it appears that this balloon is considerably larger than typical experimental weather balloons. Why did he need such a large balloon that had a cabin?
The guy is an inventor, so it's not out of the realm of belief to think he was trying to build a better weather balloon. Plus, if you're doing it on a home budget, you have to account for not being able to purchase materials that the Navy could, so that will change your design.