This is my version of the Weighted Companion Cube from Portal. I built the model from scratch, using photos of the cube from the game as reference, then texture mapped it to match the one from the game.
I've made it into a hollow box, which you can use to store small stuff, like keys, secret maps, wedding rings, plutonium, ect.
You should get it printed in Color Sandstone, as it comes completely colored, no painting required! If you really want to paint it, I would suggest you get a "White, Strong, Flexible" print of my other Companion Cube model, as it makes a tougher and more easily paint-able Cube. You will need some form of Glue to put this version of the Cube together as well.
It's hard to see, but I put a faint picture of the Cake inside the box.