Pick the brains of our resident tech geek. Metalized nylon, a material similar to mylar. The thickness is 1.5 mil and they are very durable. We can custom make almost any size or shape.
Metallized nylon comes in white or silver.
Then refill with helium. DraganFly Innovations sells replacement envelopes for all of their airships. About 2-3 weeks. Helium, being such a small molecule would even seep out of the Scrooge's wallet and metalized nylon is no exception. After a couple of weeks, your envelope might be sagging, just top up as needed.