Personally, I think the barrel shaped screw in hoods are a bit too bulky for the small pancake lenses. So I prefer to use a 46mm to 37mm step down ring as a compact hood.
Below is a picture of the Sigma 19mm f/2.8 lens with the original hood (left), and 46mm to 37mm step down ring as a hood (right):
|Sigma 19mm f/2.8 with original hood||Sigma 19mm f/2.8 with 46mm to 37mm step down ring as hood|
But for keeping out light, which is better?
To try to answer this, I have superimposed images of the original hood, and the 46mm to 37mm step down ring used as a hood:
We see that the original hood keeps out light coming from more than 56° angle, while the step down ring only keeps out those coming from more than 70°. Hence, the original hood does a better job of keeping out light from outside the field of view.
I still choose to use the 46mm to 37mm step down ring as a hood, since it keeps the lens profile small. It also makes me feel safer using the lens, avoiding accidentally touching the front lens element with foreign objects. If you go this route, you will also need a new 37mm front lens cap.
I also use the 46mm to 37mm step down ring as a hood on the Lumix G 20mm f/1.7 and Lumix G 14mm f/2.5.
On the Sigma 30mm f/2.8 EX DN, I use a 46mm-28mm step down ring as a hood. Given that the Sigma 30mm lens has a smaller front lens element, and a more narrow field of view, this hood works very well. I also got a 28mm front lens cap to use with the custom hood. I don't feel the need to always use the cap, though, since the step down ring protects the front lens element quite well.
Unlike most other Panasonic zoom lenses, the compact Lumix X PZ 14-42mm f/3.5-5.6 does not come with a hood supplied. As a simple hood, I use a 37mm stand off ring, essentially a cheap 37mm filter where I removed the glass. It works fine as a protector, and does not cause any vignetting:
The Leica 45mm f/2.8 1:1 macro lens also has a quite small front lens element, so I use some stacked step down rings as a quite effective hood:
There's a 46mm stand off ring, a 46mm to 37mm step down ring, and finally a 37mm to 28mm step down ring. You could skip the 46mm stand off ring, and only use the two step down rings. That works well too.