Update (10/2017): Since the answer below was written I have used a 3rd party hood bought through amazon that, although made of black plastic, had a shiny, reflective finish. If there were strong light sources in the actual field of view, it actually made flare worse than using no hood at all. The light was reflecting off the inner surface of the hood and into the lens. Since the hood was made for a 24-105mm zoom lens, at longer focal lengths light that was not even in the frame but was within the "cone" of the hood's coverage could cause additional flare due to reflections from the inner surface of the hood. The fit was fine and it stayed in place better than the original, but the finish was too shiny.

I just superglued the broken original hood lined with black flocking back together and placed some duct tape on the outside of the crack in the hood. At some point I plan to get some flat black model paint (used to paint scale model plastic airplanes, cars, etc.) and see if that will eliminate the reflections. Another possibility is getting some black flocking material at a craft store and lining the generic hood with it.

Further Update: After being repaired for the third or fourth time, the original cracked hood refused to stay glued/taped together any more.

I wound up getting a sheet of Creatology™ Peel & Stick Felt for about $2 at a local craft store. I made a pattern out of thin cardstock by tracing the outline of the cheap knockoff hood as I rolled it across the cardstock and then trimming it until it would fit inside the slightly smaller inner side of the hood. Once I had a pattern, I cut a piece of the felt to fit, peeled off the backing and used the self-adhesive on the back of the felt to stick it to the inside of the cheap knock-off hood. It now works like a charm!

I've used knock off hoods for many of my Canon lenses that weren't supplied with one. I've never had a problem with fit or vignetting, at least not due to the hood. Some of those lenses had so much peripheral illumination drop-off you might as well call it vignetting! I've always bought them from fairly well known dealers such as Adorama or through amazon.com

The only place where the factory hoods are a little better is that the higher end ones that usually come supplied with Canon's "L" lenses are lined with a fuzzy material that helps trap dust before it reaches the front element. The flocking also inhibits reflections from the inner surface of the hood entering the lens and causing flare.