Woodworker extraordinaire Jason Gardner (EpicWood) constructed this Portal Companion Cube Storage Box and is on sale for $500 on Etsy.
“One of the center circles comes off, allowing you to put things inside. This is furniture grade and very strong. You can sit on it, if you wish. It is also pretty heavy (about 20 lbs). It’s big too! What I’d imagine to be actual size when compared to Chell in the game. All six sides are finished.”
Check out more specs / design information here.