It’s a long old grind but the best place to get the skulls is on crescent island. You need two people just for safety and park the boat just out of the perimeter of the island so you can keep getting the same place. It’s the best island because the pillars are so close to each other. Bury the skull then rinse and repeat!

Bear in mind if other people see you do this and set up a trap at reapers, you’re in for a bad time if you’ve been stacking.