I have been making Surplus 20mm Ammo Cans into 'legal' storage containers for my Propellants here in California.
I have been using 1/2" nominal Marine Plywood to add two layers of fire resistant liner to the inside of the 20mm Ammo cans. This leaves room for at least 5, one pound powder canisters standing in the bottom an one or two laying on top of those in each modified Ammo can.
Also one pound Black Powder rectangular canisters fit three abrest across th enarrow inside dimension and three rows along the longer dimension with some room left over for another canister at the end and space above them for a few others laying on their sides.
Here in my City, County, and State the Home storage limit is about 25lb of Black Powder and some similar limit for Smokeless Propellants when stored in fire resistant containers.
I also store primers separately in a plywood lined 'Fat 50 cal' Ammo can.
These containers are NOT Stored inside my Home, but are secured in a separate shed in my yard.
Chev. William