Actually, I use something better: gaffer's tape. Electricians on TV and film productions use it to hold stuff together and tape down wires. It's way stronger than duct tape, but doesn't leave sticky crap all over everything when you remove it. It works well for taping reflectors, flags or scrims in front of lighting when you don't have a stand handy. In a pinch, you can make a flag or a barn door out of the stuff.
It used to only come in matte black, but now it comes in other colors, like chromakey green and blue or white and yellow for striping.
I keep a roll under the rear seat. I don't put it in the bag because it would be covered in fluff from the bag liner and the edges are too sticky to easily slide it in and out of a plastic bag.
It's pricey, though. For the next best thing, Gorilla tape is pretty good, too.