Method 1: (Takes 60 seconds)The following skull can be picked up in Forge mode on the map
Sandbox.
From the middle of the map, you should be able to see the sun. To the right of the sun, you'll see two pillars, and right between these pillars, at the very outside edge of the map, you will be able to find the skull. Use forge mode to spawn a "Tube, Corner" in the Scenery category on the map. Now go inside the corner, pick it up, and fly over to the skull's location, using the piece as protection from the laser towers that will try and kill you once you leave the boundaries of the map. You will be able to see out of the tube, so flying there shouldn't be an issue. Once you get close to the skull, place the piece on the ground, let it go, spawn inside and pick up the skull!
It can be seen in the video below, and I have included a timestamp of its exact location:
[
1:30] - Skull 3 - Sandbox
Method 2: (File Required)Go to the file share of "mouse6666" and download his sandbox skull map. Once downloaded, just load the map, walk through the teleporter and hold the button prompt to pick it up as you pass through.
Both methods takes basically the same amount of time and effort after you factor in the extra time spent finding the required file, and selecting the map variant.