Yes, but also no?
I think that the only good way to approach this was already stated by [MENTION=1465]Li Shenron[/MENTION] - "I do nothing, because I do not play an economy simulation game, I play a game of adventures."
"The best way to approach it is to not approach it," is kind of a discussion-ending, non-starter of a comment, though. It is kind of like walking into a discussion about making peanut-butter sandwiches, and stolidly proclaiming, "I am allergic to peanut butter!" I mean, sure. Fine. That's great. But you're kinda done once you've said that.
I mean, I happen to agree with you that armor *as we currently think about it* is bespoke- but then again, so was everything back then, because there was no technology (or magic) to make it quickly.
It is not enough for production to be quick. If you want to get away from bespoke, you need quick, cheap, and repeatable - so that you can produce an array of items with minor variations, which get close enough to the customer's needs that you don't actually have to talk to the customer.
We accept off-the-rack clothes because there's a vast supply of them in a variety of sizes and styles, so that we can each find the one that fits us well enough, that looks good enough. You may need dozens of different items to be able to make sure the customer wants one of them. If all you get is quick, but not cheap, then you, the producer, cannot afford to produce a wide array of items beforehand.
A bog standard 7th level wizard can cast a spell that takes 10 minutes, but can do it only once per long rest. That's faster than a smith can do it, sure. But, it isn't what we think of as "mass production" levels either. How many suits of armor can you sell in one town? Do you spend a month making 30 suits in a variety of shapes and sizes and styles, hold them in stock (which has a cost associated with it - space to store them, someone to polish them up (because rust will happen if they are not maintained) and hope they sell? Or do you wait for a customer comes in and "fabricate on demand"?