Back when Bungie owned Halo you could go onto bungie.net and download custom forge maps. Does anyone have one for Halo 3 to get the sandbox skull?
If you can’t find any, you can always just overload the map to get it, or hold a double wall or something big to protect you from the shots while you fly over there, or build a giant tunnel for fun 
Just overload the map.
I overloaded the map, with ease.
I have one. Look in my fileshare. All you have to do is go in the teleporter.
Don’t waste time overloading the map, just grab a large enclosed y-piece, situate yourself inside of it, and fly towards the skull, if you’re inside of it you are fully protected. That’s how I did it back in the day.