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I carry the Dan Signal Surface Kit . It is 6' L X 7 1/2" W and comes with a whistle, signalling mirror, and cyalume stick, and can be filled orally, with reg, or by LPI. It also has a dump/pressure relief valve.
Both my wife and I use these on all boat dives. Well made and large enough to be seen from a distance.
I see no reason whatsoever to buy a cheap SMB. My (I hesitate to be overly dramatic by saying life) comfort, both mental and physical, depend on this device - no reason to be cheap.