Case for ZS20
while it does not meet your requirements, as it has a zipper, I'm a big fan of the Lowepro Rezo 30 case for ZS cameras. I originally thought the zipper would be a pain, but I've found it to be a non-issue. (I can keep one side of the zipper "open" and the other covering the top of the camera and very quickly bring the camera to bear.)
Although I carry the camera in this case always, I only keep it on a belt when I'm hiking. Otherwise it is in a waterbottle pocket of my daypack.
The Lowepro has quite a bit of padding, without being overly bulky. Its belt strap is quite good- it velcros open, but has a back-up snap for security. The secondary pocket holds a battery, but is better with just a second memory card. Although it is most often in black, I tend to like the colored ones- and look for them on the cheap on ebay.
Good luck with your quest.
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