Saving a "loot pool" item

Ok, so I am making a barrel GUI. One item is determined by a loot pool. However, when players put the wrong item into a slot, it deletes all ui items, finds the incorrect item, and puts it in a "trash" slot so the player can reclaim it. It then puts the GUI back together. Except the one UI item. Because the ui item comes from a loot pool, I need to know how to store that specific item (its in container.4), and reference that in an item replace. One idea I had was to use container.27 as a temp storage since barrels don't have it, but it didn't work. Any ideas?