https://www.youtube.com/watch?v=1fmdpplbvf0
For those of you who enjoy video content and just want me to talk to you in the background or a second monitor while you play, then that's there. And here below is a text list of the 10 ways I know to get once daily gold Plunder Skulls and I am 99.9% sure there is no way for more. 

In the video I also show loot for 10 Goldens and 39 Epic openings. The loot was pretty lackluster but I did get a
Dremora Style Motiff Book worth a few hundred thousand gold.
From the gold PS's.
1st Plunder Skull - Any Incursion (Dark Anchors, Dragons, Volcanic vents, Mirrormoors Mosaics and Siege Camps, Geysers, Harrowstorms)
2nd Plunder Skull - Any Normal Or Vet Dungeon Final Boss
3rd Plunder Skull - Public Dungeon Boss
4th Plunder Skull - Delve Boss
5th Plunder Skull - Infinite Archive (Tho'at Replicanum after killing 4 of the smaller bosses along the way)
6th Plunder Skull - Any World Boss's
7th Plunder Skull - Any Solo Arena Final Boss (Final boss in Maelstrom Arena, Blackrose Prison, Vateshran, Dragonstar Arena)
8th Plunder Skull - Any Trial Final Boss
9th Plunder Skull - Kill Crowbourne Horror (Quest at the Witches Coven)
10th Plunder Skull - Kill Lord Hollowjack (Quest at the Witches Coven)
I wish you all the best of luck, happy trick or treating!!
-ESO Feanor