Gerber (and a couple others) make rigid baby bottles that are a bit bigger than the average closed fist. They withstand heat, and have a reduce radius around the middle, giving them a 'waist' that makes them easy to locate with zipties. They've proven crash worthy in my use. Lightly fill with air filter foam, and if you can see any sort of high tide line, it's time to empty them as you've overwhelmed the foam's ability to keep things suspended.