An old paint can lid, two punches with a nail, a couple of twists of the wire:
A baked beans can top, done up the same way:
This strikes me as very workable. They should be easily movable and weatherproof, should interchange easily as they get shot up (note the two minor dents in the second picture, a test pair from the green-gas 1911 from very close--maybe three yards), and should respond well to spraypaint when visibility or contrast is required.
They're inexpensive, obviously, but they solve some other problems as well. Ricochet is much, much better than with heavy steel (e.g., steel spinners for .22 rimfire) or other fixed surfaces. With the baling wire attached at two points, they actually seem to strike a nice balance between reactive feedback (the ping sound and movement) and not spinning so much that you can't put a pair on it at speed. The wire loops should be friendly to tree-branch attachment, without being so heavy or tight that it would dig into the tree. And of course there are all kinds of sizes for steel can lids. (The paint can lid, actually, produces a most satisfying sound on being hit, and being larger can be used at greater distance with normal perspective, rather than being used at closer distance with reduced perspective.)
With a little more thought, I wonder if I could get even fancier with the concept. Different-colored disks could be brought together for shoot/no shoot scenarios, or maybe I could fashion Airsoft Pepper Poppers (maybe by tin-snipping open the can itself, and using the whole thing) and learn how to calibrate them for a given power level. I hadn't thought about this before now, but if ranges are kept Airsoft short, target resets for thing like falling Poppers may not be nearly so annoying. :-)
Their big dependency, really, is that they presume that the BBs really are biodegradeable. I'm still running my first test on that, but if these things really do break down reasonably well, this may be a great poor man's solution to fancy reactive targets.
Next: get in a few serious testing sessions, and see how they hold up.