I used to get 35mm film containers free from photo developers which were handy for pre-measured powder and shot + wads, though I guess they don’t develop many films these days....
I also have a shot flask. If using a powder flask I never pour straight into the barrel, I decant it into a brass adjustable measure first , then put it in the barrel. Unfortunately I don’t shoot M/L shotgun as much as I once did. If you’re after anything let me know - I have loads of shot, wads, and the odd measure and gun....