I think the dichotomy between short-rest-based and long-rest-based classes already leads to some pacing weirdness in the adventuring sequence, so I'd suggest that adding another layer of timing for a character to ''do their thing'' would not be such a good thing.
''Ok guys, can we take 10 days off so I could forge an adamantium dagger before we go rescue the chickens from the Court of the Mad Golem King.''
I think crafting should be counted in terms of long rests that do not need to be sequential.
Ex: crafting my adamantium daggers requires 1000 gp of material (rare minerals), proficiency in and access to smith's tools and 4 long rests without any other activities. I can still adventure in the days, and use my off-screen resting time to create the item. It could requires a test at the end of the long rest to see if progress is made.
I can appreciate this, but that particular example would be blisteringly fast.
I would like a table of items, cost and rarity of raw materials, minimum skill (crafting steel vs. adamant, &c.), and time required. Ideally these times would be informed by real-world artisan skills when possible. I would tend to have this activity happen during downtime, but some time during long rests could be done. (Assuming that a "long rest" isn't just sleep and recovering from the day. Otherwise it would occur during the 12 hrs or so that includes the long rest.)
As an amateur blacksmith, I can crank out an utilitarian dagger in a week / 30 hrs. The first 8-10 hrs will require a forge, the rest will be filing, drilling, and carving. The first bit I couldn't do on the trail, but the last part would be possible. If you wanted it to be pretty, that would take longer; as long as the actual production of the piece, actually.
As for actual products skills, probably mostly smithing, weaving, shipwright, carpentry, masonry, jewelry, visual artistry, and musical composition. That's what I can see needing for strongholds, magic item components, and other cool things characters might want to make for themselves and their team. Low level consumables such as minor potions, medicines, acids, &c. would also be useful.