My old powder cabinet was built with 1 1/8th plywood, now I keep it in an upper kitchen type cabinet out in the gun room (detached garage). Major temp swings are not good, but moisture/ humidity is much worse. Even in your shop, I'd suggest building a wood box, line it with 2" rigid insulation and leave it low to the floor. Or just get a big plastic tote with a rubber seal, line it or a cooler and throw some desiccant in it for moisture control.

What about using an old refrigerator?

I store most of my powders in 1 gallon black, UV resistant, jugs. They hold around 7-8 pounds each. I transfer my multiple one pound jugs to the 1 gallon containers. This helps to eliminate the "lot variation" issues. These black jugs seal up very well and I have had zero issues over the years.

Another benefit is being that all of the jugs are now the same shape, it makes it much easier to store.

I like the "lot variation" issue resolution by combining all of the 1 pounders.

Where do you get the black 1 gallon jugs? Got a picture?

I probably have 20 empties left.
You can buy them fron ULine however I believe I had to buy 80 jugs...lol
Maybe it has changed by now.
I can GIVE you 4 if that would help you out.

In the manufacturers container that it was meant to be stored in. Not rocket science. Away from direct sunlight or heat source and good for years. If you have doubt read the manufactures recommendation, they know a hell of a lot more than then users. Why does everyone want to put a complicated twist on something so simple?