Go to a bike shop and ask if they have any old mountain bike inner-tube lying around. It's usually thrown out or given to ingenious DIYers like yourself free of charge and it can be used in so many different applications related to scuba diving you'll marvel that you never thought of it sooner.
Use a whole piece (still connected around) about an inch or less to secure a backup light, use long even pieces as straps for your knife, etc. Inner tube from a bike tire okinawascuba, you'll love it. I assume you're close to a US base but from what I remember of Okinawa bigger sized bike tubes shouldn't be hard to come by. Street bikes with the "fat boy" tires have tubes that work about as well as mountain bikes, fyi.
