I'm up to stacks of 16 of the keys now, on multiple accounts... I'm using the Captain's Airship Pass and gliding over to the quaggan Mellaggan Shrine, perhaps mining a little iron along the way. Or if it's convenient in world versus world, sometimes I kill ten of the krait there, on the Eternal Battlegrounds next to the hylek village. I wish they respawned all the time, rather than their area being unoccupied for part of the cycle... It would be helpful if, when the devs manage to sort their mistake out, we can use all our stack of keys at one chest!