Solution edit: As of 13th January 2022, the majority of online services for this game have now been closed so searching for the skull maps via my file share will no longer work. I have uploaded an alternate guide for the Sandbox Skull below.
For sandbox, you will have to overload the map with trip mines in order to access areas out of bound to locate the skull. On top of red base, spawn a trip mine and change the summary settings to 10 second respawn and run time minimum 1. Stand on top of the trip mine spawn, then keep releasing them and make a large pile without moving your player. Once successful the map will start to glitch and delete objects indicating it is now safe to access the skull.
The remaining skulls can be found using the below video guide.