In rule 2.10 Inventory add the text
Items may have no more than one Holder each.
Create a subrule of rule 2.5.2 Master Thief, titled “Rewards”, which reads as follows:
The Survivor who fought last with the Boss and caused its HP to drop to 0 may once become Holder of one Item of his choice from the “Inventory Items” page.
First one so that we don’t have loads of items around, second as a way to get them out into the world. What a great and legal commentary/flavour entry.