this is a pic i already have of the fence 95% complete --- the only thing not done in this pic is securing the netting to the fence. we did this with slats of wood -- in a possible fit of overdesigning, i wanted an easy way to clear debris from the netting (leaves, etc). i figured i could just remove the slat, everything would fall through, etc. however, i think were i to do it again i wouldn't worry about this... it would be easy enough to clear debris with a small ladder and just reach over the netting
materials:
1/2" pvc pipe (pretty cheap in 10 foot lengths, Home Depot had a tool available for use that let me easily cut it right there). the 45 degree angler also came from plumbing.
The brackets to hold it to the fence came from electrical (used for conduit).
If you look close, there is a another little metal thing at the top of the pipe -- that's also found in the electrical, and screws tight onto the pipe to give me something to connect the netting to.
the netting is connected with standard plastic cable ties. we bought them in bulk for cheap at monoprice.com (the best and cheapest place for any sort of cables, by the way) though you can buy them at any hardware store. one at the top, one in the middle, one at the bottom. you can also add one if the netting sags over time to 'tighten' it.
we didn't paint the pipe, but that could be done easily enough.
from our research, it essentially creates an angled surface that animals can't leap over, and don't feel comfortable walking on (the 'give' of the netting actually works in our favor).
We placed the posts at about 5 foot intervals.
pipe was about $2 per 10 foot piece (far cheaper than buying them in 2 foot precut lengths). the various brackets and such were all in the 50 cent range... 30 cents here, 60 there, etc.
We did NOT glue the pipe in, and i'm not sure i would were i to do it again. yes, we had some early breakins where something pulled out the pipe, but a tiny patch of hardware mesh over the connection seems to have done the trick. it's a diy project, so i'm guessing anyone who does something like this will add their own ideas/improvements.
We also had a section towards the front of the house that was shorter (10 feet) and my wife didn't want the poles to be visible, so for that shorter length i simply mounted a metal wire about 3 feet from (and level to) the fence and ran netting from the fence to the cable. that creates the same barrier, but is invisible, so passerby won't see it. but i don't think this would be feasible (at all) for the 40foot fence we had to do.
The netting is nearly invisible, so isn't an eyesore. painting the pipe a neutral color (or black) would also make them less visible to neighbors, etc, but that wasn't a concern given our situation. (and really, since they lean 'in' to our yard, they aren't that visible from our neighbors yard anyway.
hope this helps someone! it's worked out extremely well for us, both in critter prevention and cat retention. again, have a secure coop since i'm sure an industrious raccoon could get in the run if it really wanted to, but i think we've created something that's so annoying to them they don't bother.
we were initially inspired by the 45-degree angled sections on top of barbed wire fences, and after doing some research we found some more "official" cat containment systems (such as catfencein.com) but they were way out of our price range and not quite diy enough for us.