(I'm just going to keep posting here, since this thread links to that one, but not vice- versa, if this is a problem for you feel free to bite my head off. I'm assuming at this point that you have read both threads.)
How about, structural degradation for Poor quality bridges? The caravan camels are the straw that broke the camel's back. When they step out on the listing, creaking wooden bridge, a support beam snaps, and the entire thing crumbles into bits.
You fish the elven goods out of the dry canyon, and build another bridge that won't really last through winter.
I don't feel that any dwarf, no matter how unskilled should fail to make a table that can be eaten off of. The thing should be ugly, wobbly and hard to clean, but you can theoretically balance a plate on top of an unshaped rock.
A lot of the quibbling in this issue seems to grow out of different ideas about how much stone is in a unit of usable stone. The answer is: enough to make one table, one piccolo, one wall, one ring or three mugs.
Any unit of anything you can build a table out of is worth more than one ring, by volume, so some amount of material waste must be abstracted.
I'd like to see waste have more to do with skill than what you are producing, so that better craftsdwarves got more rings out of a unit of rock, instead of mugs just being better than everything else.
Even having never done it before, I bet I could get more than one toy boat out of a whole tree. They might even float right side up.
I would take forever, and I'd spatter the entire vicinity with powdered wood, but I'd do it.
I'd probably get a boat out of every log in a unit of "wood logs", if the definition of boat is sufficiently loose.