Warhead has just uploaded Ultimate Boss Chest!
This mod overrides most/all Boss' Chests spawnclasses because the original one wasn't "rewardable" enough. In fact, most Boss' Chest with the exception of The Forgotten Chest (Chillhoof in the Icedeep Caverns) and the one from the Spider Cave, didn't even have the proper spawnclass. Most of them called 'Treasure_Chest_Huge' instead of 'Treasure_Chest_Boss' as spawnclass.
So, I re-used an old one... (continued here)