I sketched out a rigid design once using sturdy board (foam sandwiched in paper) for the internal frame. It's easy to work with and gives a strong yet springy frame. Mylar for both the external skin and the gasbag(s). Figured it would have to be about 12 feet long to get a decent payload. Airships don't scale down well. Made a few small non-floating prototypes, lost interest when I couldn't find a sponsor. Helium is pricey!