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I have Manfrotto, Lastolite, and Westcott brackets, as well as lots of cheaper ones that I don't like as well. It may seem overkill to pay $30 or more far a swivel adapter, but the good ones are really worth it in the long run.
Some rely only on friction to hold the set angle, but the Westcott ones have ratchet "teeth" that firmly lock with just a little pressure.
Lastolite has a similar ratcheting adaqpter:
I bought a Lastolite Brolly Grip, and while I like the way it keeps the Speedlite close to the lens axis, it lacks a locking screw for the umbrella shaft and so the umbrella rotates and can even slide out of the mount.
Unless you don't mind using gaffer's tape or extra clamps of some kind I can't really recommend that one.
One of my favorites is the Manfrotto: it holds securely, comes with reversible spigots for a variety of uses, and is made of rugged metal. There are a lot of copies of this one, some good and some bad, but the true Manfrotto one is great.
I also have a specialized one that holds two Speedlites at once, as well as flash brackets for my beauty dish and so on, but these are the basics.