There may be a simpler way I'm not familiar with, but you just need to use a bit of math. If you had a 200 pixel square comp, and stuck the null in the dead center, it's got a position of 100,100,0. 100px-wide squares would need to be arranged around that origin point—the "front" face would be at 100,100,-50, for instance, the back at 100,100,-100, one of the side faces would be rotated 90 degrees on the Y axis and placed at 100,100,-50, etc. Snapping and an easy way of adjusting anchor points for rotation such as the RepositionAnchorPoint script make it trivial to quickly do this; then you parent it to that null.
Note that you will sometimes get some fringes and gaps/seams in your cube, even if mathematically it's perfect. Setting the cube faces' blending mode to "Alpha Add" usually helps with that.