"The Weighted Companion Cube (often referred to as the Companion Cube) is a recurring object in the Portal universe. It appears almost identical to the Weighted Storage Cube but with a pink heart replacing the Aperture Science logo." -ThePortalWiki
-Cardboard (Cereal Boxes Work Great!)
-Box Cutter/ XACTO Knife
*The Weighted Companion Cube and all other references from Portal and Portal 2 belong to Valve Corporations. I do not own the shown icon.*
Step 1: The Body
First, draw out this shape. The six squares are 5cm x 5cm, and the side flaps are all 2.5cm x5cm. Cut out the cardboard base, and, using the paper base, mark all of the creases with a pencil. Then, take your knife and cut along all of the creases. Don't cut all the way through; just deep enough to make folding the cardboard easier! This is called scoring; remember that!
After preparing the cardboard, glue the side flaps to their corresponding parts. For example, Flap A is glued to Space A. But, before folding Flaps E, F, and G, use your paper and fill the cube. The paper will help the cube hold it's shape and give the whole thing some weight.
Now, glue Flaps E, F, and G to their corresponding spaces, and let dry. After that, you're done with the body.
Step 2: Details
For the details, I drew out three different shapes: a rectangle, a circle, and a semi-square with 1/4 missing. You will need 8 semi-squares, 12 rectangles, and 6 circles.
Once you have all of your shapes together, set the semi-squares to the side. We'll get to those later; first, I'll show you what to do with the circles and the rectangles.
First, grab your ruler and find the halfway point of each side on your top face. Use these to draw two intersecting lines, and repeat this on all six sides, as shown. Now, use these lines as a guide to glue on your rectangles and circles. Take all of your rectangles and turn them to a landscape (longways) view. Then score a straight line across the center of each, and fold each piece in half. this will make it easier to wrap the rectangles around the cube so they fit perfectly on the clean side of the box. Glue the circles in the center of all of the rectangles. I recommend doing the circles after the semi-squares to ensure that they are centered on each face.
Now, back to the semi-squares. Use your knife and score along the lines given, as shown above in the third picture. Don't cut all the way through; this is for folding again! Next, connect the now three-sectioned piece to itself so it makes a pyramid shape, and slide it onto a corner. Repeat for the remaining semi-squares.
After, the semi-squares dried, I went back and added some final touches, such as outlining pieces and adding hearts. Then, you're done!! You now have your own miniature Companion Cube!
Step 3: Cube Customization
Right now, I am working on a printable version of the Companion Cube. Once that's done, I'll make an updated Instructable with the attached files!
If you want your cube to be a different color, I can definitely change the bases to your desires! Comment below with your email address and color (a sample or picture reference of the color would be extremely helpful) and I'll send you the files with the new colors.