Been back there for a couple levels now with my 51 druid and 52 rouge... Saw this drop after killing at least 300 aliens. It is extremely rare, but it was off one of the standard Grol Bakus - even checked my logs to see if it was a named, but no.
Hoping I'll get one of those weight reduction boxes before I leave.
I've been back there for some time now with my 51 druid and 52 rogue... I'd say it could be done with a 45+ duo. Fairly easy to get single pulls with a bow.
It is, however, an ultra rare drop. I've killed around 400+ aliens, mostly the highest level ones (blue/lt blue to 52), and seen this once. Just hoping I'll see the 10 slot box drop before I leave.
In the cave behind the waterfall of the secret valley. It was a rare rare drop. Another drop I got in this location was called a Bottomless Box which was a 10 slot weight reduction box 100percent that weighed .1. Got a ton of Aqua Shields off the named that spawn in the cave called Aqualung Deepwater or something similar. The amice and box were just rares off normal mobs back there.