A safe border crossing point so that we can fetch ice without risk.
That's the actual thing that we could develop and which would be valuable. Instead of running for the border and hoping for the best like we've been doing, we could find some way through that's reliably safe and can be used repeatedly.
In fact, that would be a smart thing to do for every border we can. Border crossings are the point at which we're in the most danger. Having a reliable, low-risk way to get across borders would be to our benefit in virtually any plan or endeavor we pursue from this point forward. Put your collective genius to work on that!
Whether it be a carefully dug tunnel or some sealing trick or set of well-established false identities that can survive scrutiny by bordr guards, we ought to come up with something.
We should look into the migratory flight paths of birds in various nations so that we once we have a zeppelin we can follow the same routes, just at a much higher altitude to appear to be the same size as a bird from the ground. With reliable air travel we can cross any border (well, maybe not extremely high mountains) with ease. Zeppelins were once used to transport a variety of goods and people once upon a time, but due to the costs of operating and maintaining them, fell out of favor (especially after the Hindenburg tragedy).
However, we have physics defying magical bullshit superpowers.
It doesn't matter how heavy our cargo is, since we can store literally tons of goods in storage scrolls for a fraction of the actual weight of said goods. We can have a non-flammable source of lift in the form of Elemental Mastery (preferably used in Seal form to prevent Akane from tiring out while we fly). We can direct our zeppelin with custom-made thrust seals (which would simply be explosive seals that explode slowly over time and only in one direction, just like a rocket engine). Or maybe we can ask Mari for a more suitable wind technique.
For ballast, we can also use storage scroll as other people have mentioned: seal heavy thing to go up, unseal to go down. Use multiple ballast seals to control the acceleration as needed.
At high altitudes we will have to maintain cabin pressure and temperature, but hey we already have Thermostat no Jutsu.
The hardest part will be finding a suitable canvas, but
this site claims that silk or
paper with linen was used initially for the envelope (the term used for the outer material of an airship). We might also be able to make an altered barrier seal that acts as a bubble as well if we want to make something more reliable, unless of course we want to invent synthetic fabrics.
Note that a proper airship has a variety of smaller bladders housed inside of the outer covering to ensure that one puncture will not spell disaster for the whole ship. Redundancies are a good thing because they make you safer. No need to go all-in on anything when risking your life; hedge your bets.
Which still leave a whole bunch of other issue to deal with, such as recruiting and paying reliable agents, getting paid our share of the fee, or what have you.
We should be the only ones involved in gathering (or creating) and transporting the ice. The only people we interact with are the ones buying the ice and we go in henge with masks underneath. We can get paid on delivery.
It doesn't need to be complicated at this point since we are just getting started.
