one handed, with your arm against your side and the camera against your forehead. It quite easy really.
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Your hand is 1 stop brighter than an 18% grey card, so it can reflect a lot of light into the lens. Your method could also be awkward and unstable, as you'd have to put your hand in different locations depending upon where the sun is. It might have to be on the RIGHT side of the camera, for example. All that to save $5-20 on a hood?
As for third-party hoods, why not, especially for the bayonet-mount type? I have several that work quite well, including hoods for the 17-55mm f/2.8 IS, 17-85mm IS, 18-55 non-IS kit lens, and an ET-67B to use with the 100mm f/2.8 Macro. (It's shorter than the "standard" ET-67
The only third-party hood I've had problems with was a copy of an ET-65 III (fits 85mm f/1.8, 100mm f/2, and 135mm f/2.8, all of which I have). That hood uses spring-loaded fingers, rather than a bayonet mount. The one I got seemed to have either a weak spring or, more likely, one finger that was too small. I was able to find a Canon ET-65 III on eBay for about $20.
Edit: While you can get third-party hoods at B&H, Adorama, etc., you might save a bit by buying on eBay from one of the many Chinese sellers. (My "favorite" is jiakgong.) It can take 2-3 weeks to arrive in the US. (I don't know about other countries.)