Coffer Keys: In all likelihood, you will have spent at least some time between levels 50-53 experiencing off of Helm Beetles. Monk, Beastmaster, White Mage, Bard, and Ranger also need this coffer for Artifact equipment, but I spent so much time there that in almost every party, we were throwing keys away because we all had them already or didn’t want them. If you don’t get them in an experience party, three or four members level 56 or higher on Helm Beetles should be fairly safe. Should Helm Beetles not be your flavor, coffer keys also drop off of Knight Crawlers, Crawler Hunters, Wespes, and Water Elementals.
Coffer Spawns: Again, it is fairly easy to solo retrieving the coffer itself with some manner for self Sneak/Invisible'ing. I spent well over an hour camping the two spawns where most parties exp on Rumble Crawlers and Helm Beetles to no avail, went back and down to the second map and found the coffer waiting for me. I realize not many people are as crazy as I am about quests, and will not have done the 15-part map quest for Crawler’s Nest, so I will try to explain which areas are “maps” so you can seek off of the appropriate coordinates. The First Map encompasses the main entrance, and the passages from and to the first main open room, tunnels off of which players will experience off of Worker Crawlers. There are no coffer spawns on this map. The Second Map is reached via the North-Westernmost passage off of the first main room (not the dead-end room with the Doom Scorpions, there should be Worker Crawlers and then a Scorpion or two in the tunnel) the spawns on this map are at F-6 (x2), G-7, and G-8 (x2). The Third Map consists of the large, square room that houses Soldier Crawlers, Hornflies, Exorays, and more. From the opening into the room, head west until you reach the far wall, and follow the outside (west) wall south until you reach the semi-hidden (well camouflaged, really) opening to a western tunnel. There are two spawns in this tunnel, G-9 on the south wall of the Rumble Crawler Room, and F-9 at the corner intersection of the Helm Beetle/Rumble Crawler area.