This is a Weighted Companion Cube from the portal series of games. I designed one side from scratch based off of pictures so it may not be 100% accurate to the game file, but whatever. I originally made this for a friend, but want to share it with everyone.

The STL file is only one side of the cube, so you will have to print six of them and glue them together.

Sanding on the faces to be glued together is probably needed and possibly a bit of gap filling along the gued edges. Prime, paint, and weather to your heart's content.

My example was printed in PLA from Protoparadigm at 0.08mm layer height on a Prusia i3 Mk2/s with 30% infill

This file prints a 75mm companion cube once all six pieces are printed.