OK, after chatting with others on DF, here's what i MIGHT go with for a proposal if i eventually decide to allow this sort of armor..
Animal Chitin armor.
Certain tough monsters such as Umber hulks, hook horrors, giant crabs and scorpions can have their left over chitin made into a form of scale or plate mail, that grants the wearer an AC equivalent to 2 worse than what the monster has, but they are only half as 'encumbering' as that sort of armor would normally be. Additionally since they are 'natural' rangers and druids have no issues wearing them. However, since Chitin breaks down when its no longer on a live host, the armor will last (without magical or other preparations) for d4+3 months before becoming useless. Additionally, every time a monster which has a 17 or greater strength scores a critical hit against someone wearing said armor, it loses 1 point of AC. And when it hits AC 9, the suit crumbles...
Once it starts wearing out, not even a mending spell can make it hole again.
It takes 3 weeks per AC point the armor would grant, for a master Armorer (someone with at least 4 NWPs spent into armorer), to craft a suit of armor, and the person it's being made for must be present for 2 days per week, much like getting fitted for field plate or full plate. It costs 50gp per point of AC the armor will grant, for the armorer to make this suit, plus a variable amount of 50-100gp (4+d6x10)..
Sample suits of armor - Giant scorpion. Normal AC 3, created armor AC 5. Costs 250gp + 50-100gp. Takes 15 weeks.
Hook Horror, normal AC 3, created 5, cost and time to make as above.
Umber hulk, normal AC 2, Created AC 4, cost 300gp base, 18 weeks creation time.