There are two other commercially available products in North America, the Flash Flag and The Whizzzz Windmill Bicycle Reflector. Both can be folded flat against the bike when you do not want them extended.

I've used the Flash Flag and although the vendor unfortunately has medical problems at present, he is continuing to sell the Flash Flag:

The flag is also reflective for night riding and wraps around the flagpole, permitting it to spin 360 degrees, completely around the flagpole. The compression spring connection at the base results in the entire flagpole oscillating about while you bike, perhaps attracting attention The spring also allows you to use the hook on the flagpole to clip it out of the way when you do not want to use it. One time the oscillations resulted in my flagpole working itself off the spring, so I'd check the connection or add some tape or a screw to secure the flagpole to the spring.

The Whizzzz Windmill Bicycle Reflector is available through Amazon and maybe elsewhere. I have not used it, but the spinning reflector might be conspicuous at night:
And if one isn't enough, mount three of them^.

These type of flags are termed lateral or horizontal flags or spacer bars in the bicycling literature. In Swedish, varselpinne or stingpinne LINK (http://sv.wikipedia.org/wiki/Varselpinne).