UPDATE 06 Oct 2010
The basic framework is up, using 1/2" EMT electrical conduit posts to support a 14' x 15' grid of rebar tie-wire, which will hold up the plastic bird netting. (Post spacing is 15 feet on center down the length of the enclosure, and 14 feet on center across the width.) Please note that there are power lines and phone wires visible, which aren't part of the framework.
To avoid having anything stick up above the wire support grid (where it might snag the bird netting), I drilled four holes in each post, roughly an inch and a half from the end, so that wires could be mounted by threading them through the end of the post and back out through one of the drilled holes. To make it easier to thread the wires, I "flared," or angled, each drilled hole with an old nail-set:
To keep things as cheap as possible, I secured each wire by wrapping it a couple of times around a "hog ring", and then crimping the C-shaped ring to form a tight "O". This creates a stop, so the wire can't be pulled back through the drilled hole.
(A more expensive, but better approach, would be to use threaded cable clamps, which can easily be removed if adjustments are needed. Stainless steel cable would be better than tie wire, and abrasion to the bird netting could be reduced by sleeving 1/4" irrigation tubing over the cable.)
I measured and laid out much of the wire grid while the posts were still lying on the ground, and then raised them all up and made the final crimp-connections from a ladder. (This was necessary because I had to work around existing branches; if someone was starting with bare ground or very young trees, they could make all the connections at ground level before raising the posts.)
The bottoms of the posts simply rest on the ground and do not have to be anchored (although once I install the netting I'll put a piece of brick or tile under each post to keep it from settling into the ground); everything is held in place by the tension of the wires, which are secured to the chicken wire fence. Tension on the wire is increased by moving the bases of the angled perimeter posts closer to the fence. (Although I don't much like the look of the angled posts, I chose to install them this way due to space limitations. Doing so will allow the netting to hang straight down at the perimeter, while avoiding any "shear" stresses on the posts, which aren't strong enough to resist any significant lateral force. With this arrangement the only stress on the posts is compression, and the weight they support is quite minimal. It would have been possible to keep all the posts vertical, but I would have had to move the fence out about 3-4 more feet, and I just didn't have enough room.)
The posts that support the central ridge are 12 1/2' high, and vertical; to get the extra height I used a coupler (see below) to splice a short piece to each standard 10' length. The perimeter height is currently 9 feet (10' post at an angle), but once I install the netting, I may be able to raise this a little higher, if there's enough overlap between netting and fence to allow it.