By popular demand I made the Weighted Companion Cube from Portal.
Step 1: Materials and Tools
some sort of hardenable or bakeable clay
Silver paint (not necessary, I think it looks better than white though.)
cutting knife or blade
Step 2: Cube Beginning
Make a cube out of the clay.
Use the blade to make a plus on each side.
Step 3: Cube Edges
Make 8 small equilateral triangles.
Push the triangles onto each corner and shape them.
Make 8 small rectangles.
Push them onto the edges between the triangles.
Make 6 small balls of clay.
Squish them and push one into the center of each side of the cube.
Step 4: Paint
Bake the cube if it is a baking clay, let it dry if it is a hardening clay.
If you want the edges to be silver instead of white, spray-paint the entire cube silver. Otherwise paint the edges, corners, and circles white.
Mix the black and white paint to make a dark grey paint.
Paint the low area between the edges and the circle grey.
Mix the red and white paint to make pink paint.
Paint a heart in the center of each circle.
Step 5: Don't Let It Die
Whatever you do, don't let your companion cube die.