The following skull can be picked up in Forge mode on the map Orbital
In this almost symmetrical map, you'll want to go to the middle connecting bridges. From there you will see a large and long metal wall. While looking at this wall, if you follow up the right hand side, you will see two tiers of small platforms poking out the side of the wall. Use the forge mode to ascend to the highest platform, and you should be able to find this skull. You may have to crouch and land on a precise point in order to be able to pick it up.
Exit forge mode and make sure you pick up the skull.
It can be seen in the video below, and I have included a timestamp of its exact location:
] - Skull 2 - Orbital