Trogman,
Panasonic 18650 3400mah batteries (also known as NCR18650B's) should fit in any current Zebralight that takes 18650s. The NCR18650B is pretty much the smallest diameter and shortest length 18650 on the market because it does not have built-in circuit protection.
The reason NCR18650B's don't go in some flashlights is because these batteries don't have a button top. It's not a problem with Zebralights because they have a spring at each end on the inside, which was a good design choice on Zebra's part (see pictures of my MkI H600 below).
If there is any remaining doubt about whether or not NCR18650Bs can fit into a Zebra, you can always purchase some of their own batteries, which are actually just NCR18650Bs repackaged with a mini circuit protection board (for twice the price of an NCR18650B, might I add):
http://www.zebralight.com/ZL634-3400mAh-18650-Protected-Li-ion-Battery-ship-to-US-customers-only_p_125.htmlPersonally, I wouldn't let a couple of bad examples of Zebralight durability worry you. Every company will have its defects. I myself had a quality issue with my light, but I sent it in and they fixed it at no charge. However, I've monitored forums about Zebralight products for some time now, and I am confident that poor quality from them is not at all the norm, and Zebras are usually regarded as one of the best lights you can get for your money, both in quality and performance. I LOVE my Zebra, and when it comes time to upgrade my light I'll be going straight back to Zebralight.
By the way, if you don't care where you purchase a Zebra from, I recommend WVUnderground (
http://wvunderground.net/H600wII). It's the same price there as it is on the Zebra website, but WVUnderground is a fellow caver's website.