My one experience with a light bivy cover involved slipperiness (from the silnylon bottom) and internal condensation. The latter was probably because I toss and turn a lot, so the silnylon bottom of the bivy ended up mostly on top, where the water-repellant/breathable half is supposed to be! With some bivies, you can eliminate this problem by staking it down, but you can't do that inside a tent with a floor.

Makers of lightweight bivies (really sleeping bag covers, not stand-alone bivies) include Mountain Laurel Designs, Titanium Goat, Katabatic Gear, and probably a few others that I don't remember. The above firms will alter the bivy to your specifications (for an additional charge).

A bivy is one of the easiest "make it yourself" projects (it's basically just a bag with a zipper), if you have at least a minimal talent in that direction. If I were going to make one, I'd make it big enough so I can put my thick sleeping pad inside, too, and I'd make the whole thing out of DWR nylon (such as Momentum) instead of a silnylon bottom so it wouldn't matter if the bottom ended up on top. Your mileage, of course, may vary!

Most modern sleeping bags, at least the higher quality ones, have a good enough DWR (durable water repellent) outer shell that the bivy/sleeping bag cover isn't necessary unless you're sleeping under a small tarp. Mine has been "sprinkled" on several times when there was bad condensation in my tent and my dog started his full-body-tail-wag routine before I could wipe down the walls. I just shook it off my sleeping bag and nothing soaked in.