Alas, the party completed the Duergar section of the dungeon 2 sessions ago! :O
What about the enlarged invisible Duergar?
What we need for the Duergar is a pair of minis - one regular sized and one to represent the enlarged Duergar. Make that three minis - we also need a clear plastic one to represent an invisible Duergar.
Somebody get Reaper and/or WizKidz on the line!