I've always used the changing bag and never had a problem with it. I think the thing I do differently is inflate it with a bunch of air. It's not completely air tight, of course, but it's close enough that it'll stay semi inflated long enough to load a dozen film holders without collapsing on my work space. I had considered the PVC tent route when I first started with large format, but I quickly stumbled upon the "inflation method" and haven't found a reason to try anything else. Plus, I don't have to store PVC pipes anywhere (and I've got too much camera gear as it is). You don't want to pressurize the bag full of air, because then you'll run the risk of letting in light as the air runs out. So I just fill it up mostly full, with a bit of sag, and it works well enough for me.