The standard time to create of 1 week...out the window.
Instead, for each 8 hour period of downtime spent crafting (max 1/day), a character can make a craft check and apply the check result against the gold piece cost of the item. Once you have accumulated a total craft check meeting or exceeding the gp cost, you have successfully made the item.
These checks can represent time spent foraging for supplies as well as that spent hunched over a hot forge/alchemical lab/stove/sewing needle.
Requirements for tools will be handwaved on the understanding that crafting is only an option in a safe location where you can spend dedicated time on the process, no emergency MacGyvering/A-team/Stark-like shenanigans when you're hiding out in a shed/locked garage/cave!