Tarpaulin fasteners are a very inexpensive addition to the camera bag (10<£4!), taking up very little space, even less when looped around a handle or strap on the outside of the camera bag. If stored outside they also add an audible clacking noise if the bag is moved. (Don’t let your guard down and rely on them alone please!)
They’re extremely easy to fasten and release and can be used to strap all manor of things to each other. Such as a flash gun to an umbrella, light stands to each other then transporting kit, a camera to a car headrest as I did tonight, or a bag to a light stand to act as ballast.
They can be fastened around something small and the big loop pulled over something like a lens or battery grip. Or wrapped around something smaller and the loop at the end pulled over the ball.