Here's an actual play account of crafting from a 4e campaign:
That's exactly what I had in mind I would do if I wanted to make the crafting of an item important in a game: a Skill Challenge. This way it doesn't rely just on one roll, it asks for creativity from the players, and everyone play a part in the process instead of just standing there waiting for the smith to succeed or fail. Simple, yet fun and elegant.
Quite frankly, I don't get the obsession of needing all proficiencies of the characters written on their sheet, needing to see it written as proof that their character is proficient, especially for more artistic skills like crafting, dancing or music instrument. Why can't your character can't just be very good at it right from the start. You want your dwarven fighter to be the best smith in the region because it makes sense with his background? Why not? Characters are heroes so yeah, I expect them to be good at what they do in more mundane tasks. No need to have a skill to prove it, keep those precious skill points for skills that will actually matter in the game. Don't skill tax a player for his background.
Because honestly, why does it matter anyway? Why the need for a skill. It's not really important. Even if the player actually start a forge to sell weapons and armors, he won't make a ton of money out of it, especially compared to what he'll make adventuring... but if that's what he want to do, just go with it. Assume that he's good at what he does and has the skill to make tools, weapons and armors and sell some. Decide on an a little amount of gold he made during the downtime and move on to the next adventure. You want to make a minigame out of it to see how good it went? Why not? Maybe an Endurance check to see how much time he spent in the forge, a diplomacy check to negociate the prices with customers (if PC always negociate to buy, assume that NPC will also bargain with the PC) or just to see if someone will actually buy it (it's not because you crafted a beautiful sword that there is a demand for it, so I would argue that if a player wanted to start a buisness, diplomacy would be more important than an actual skill to create the item) It's easy to make a rule on the fly if suddenly it matter. You can just create a little downtime income table, roll on it and move on. This table could also work for the bard if he wants to spend all his time singing in the local tavern.
And that leads me to the other thing I don't get. Why do you need an actual rule for it? In most campaign, it will never matter anyway. But if you're in one of the rare campaign where it could matter, why not just make your own rule for it that actually fit your table? The game expect the DM to create stories, a world fill with multiple NPCs, monsters and big bad evil guy, different challenges and puzzles... and they deliver. So why is it so difficult to create a rule if you are one of the few that needs it? It's so easy. And it's not because there is no rule in a game that it means it doesn't support it. As far as I know, there is not really a rule for gambling, but it doesn't stop DMs to create their own mini game, mini casino. It also doesn't mean that the game doesn't support gambling. Your table, your game, your rules.