This is one of those cases where I can't help but wonder if our love of gadgets and techniques leads us to solutions that are perhaps overkill. Nothing wrong with rigging up a glass carboy, or even a plastic fermenter, to rack using CO2, of course. Mostly, I ferment in kegs anyway and use CO2. But here's what I do when my primary was in glass or plastic.
There is already a blanket of CO2 sitting on top of your beer when it's done fermenting and sitting in glass or plastic. If I'm able to siphon, I'm not really too concerned that oxygen is going to drive down through that blanket of CO2 in one or two minutes, like some sort of invasive aggressive snake, and destroy my beer. I may be wrong, and of course would love to see evidence that this happens, but I just don't experience that in my brews. I'm much more concerned about keeping it sanitary.
I use two holes in the primary lid (in plastic, I had to drill a second, smaller, hole in a spare lid used just for this purpose). In one hole goes the racking cane. In the other, some vinyl tubing attached to a sterile filter, which I blow into. Of course, the holes must be air tight for this to work.
Of course, it's important to purge your receptacle first. I fill it in well in advance with CO2, let it settle so the CO2 sits in the bottom, and then open the relief valve to (hopefully) eliminate the o2. I'm actually much more concerned about potential oxidation in this container than in the racking source, since that keg is filled with oxygen in the first place, whereas the primary is not. I doubt that the Co2 and O2 levels stratify nice and cleanly just to accomodate me, so like others said, I'll often do it twice.