Hey there,
First of all, so glad to hear that Huel has been helpful with your gastritis!! Thanks for reaching out to us with some feedback 
I totally hear you that opaque, airtight containers can be hard to find. I’m sorry that you’ve had to come up with such an elaborate solution! I think a Huel powder container is a great idea, and I’ve passed it along.
In the meantime, I wonder if you might have more luck sourcing containers by material, such as stainless steel or silicone? I think you might also have luck looking up “food grade plastic bucket” or “protein powder container” specifically. You just want something airtight, food safe, and BPA-free.
Best of luck with it!!