Add a few lens flares to give some lighting effects later on. I used 3 lens flares with 105mm prime. One of them was 100% and the other two were 50%.
Create a new layer and go to select>modify>contract and put in about 15 pixels in. Then fill the new selection on the new layer with black.
Use Gaussian blur on the new layer. Use a large enough radius that the blur almost covers the cube.
Change the blend mode to soft light.
ctrl-click the original cube layer to select the entire cube, then go to select>modify>smooth and enter 5 pixels. Then go to select>inverse. Now click on the cube layer and hit delete. This will round the edges of your cube.