Just a short note on the payload requirements. Partly in order to keep FAA out of our hair, we intend to have the payload weigh less that four pounds. Here is the exact language of the requirement:
Balloon payloads that weigh less than four pounds and have a weight/size ratio of more than three ounces per square inch on any surface of the package, determined by dividing the total weight in ounces of the payload package by the area in square inches of its smallest surface.
Earlier I had overlooked the weight/size ratio component of the rule. To provide some concrete numbers, let’s assume our payload weighs 3.5 pounds. This means that the smallest side must be, at a minimum, 3.5*16/3= 18.7 square inches. That’s 4.3 inches on a side, assuming it’s square. Now that we know this requirement, it will be easy to comply.