I am already 2 months behind schedule for mine, so I could not realistically deliver any promise. Just wanted to show you that while not simple, it is not that complicated either. But I understand that sometimes you just want to buy things, not build yourself.
with 350x350x350 you might be able to find suitable ready made box and do what ludwig_bre did for his own battery.
You actually do not need someone - there are several services even in EU
One of them - https://craftcloud3d.com/
My shell would be ~100EUR plus around 30EUR for aluminum sheets when sourced locally ( -80EUR in JLC for drilled and anodized version)
About point 6…should there not be a membrane instead (you can use that hole to waterproof-test it with an enema bulb btw;)
Battery warmth an/or environmental pressure differences need to be equalized between the box and outside. If not, significant pressure differences WILL un-waterproof your perfect airtight box. In short: Box must be waterproof but NOT airtight.
