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PVC Reducer. Navigating the PVC section of the hardware store might be a bit tricky. What you're looking for is the piece that functions to reduce the pipes from a larger circumference to a smaller circumference. The smaller circumference will be what fits around the lens barrel of your camera, while the larger circumference will comprise the “hood.”
Bring your camera with you, so that you can try fitting various reducers onto your camera to see what will fit, and what won't obstruct the periphery of your shot. Now, be aware that for some point and shoots the lens barrel will not extend far enough out to really have an effective rain hood.
Black is the best colour for the PVC, as it will cause the least corruption to any light that bounces off the inside of the reducer and into your sensor. Failing that, a little sharpie or paint can do the trick.
Sandpaper. Many reducers have bevels, which may need to be sanded down to make that fit absolutely perfect. (Note that these bevels might prevent you from fitting the reducers properly at the hardware store.) You might also need to shorten either side of the reducer so that you don't get that vignetting effect from an overprotective lens hood! If it looks like you need to do some serious material removal, it might be a good idea to get a file.
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First, sand away at the small end of the reducer until any bevels or other obstructions to fitting it onto your camera are gone. Be careful not to force the reducer onto the lens barrel, lest you knock some of guiding pins out of place. It should be a snug, but not uncomfortable, fit. You might need to do a little shaping, especially if your lens barrel has any protrusions.
Next, you'll want to sand away at any excess reducer on the big end that might cause vignetting. Test frequently by putting on the proto-lens hood and seeing how the image comes out: can you see any of the PVC? Even when you zoom in and out? Sand until it suits. It might help to tape the sandpaper down on a flat surface and grind the whole end down, just to keep it even.
After that... well, you're done!
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Using The Lens Hood
For DLSRs, there's no problem: just slip it on when you want to use it and you're good to go.
For point and shoots... you run into the problem of the lens barrel not being extended at all times. What I would recommend for this is having your camera on only when you're ready to shoot, and between shots to keep it on while the LCD screen stays off. Make sure to disable the automatic turn off, as with the lens hood on it could potentially damage the lens barrel as it attempts to retract.
It is advisable to use a rain hood in conjunction with any lens hood, that is, a plastic bag that surrounds your camera in a protective waterproof sack. This will keep your camera completely free of water and the damage it can cause.
Otherwise... this lens hood should prove perfect for keeping out the haze of the Sun out of the corner of your camera, and the rain off of your lens!
For the inspiration for this article, check out this Instructables guide for creating a lens hood for a Panasonic Lumix.