This design is a charger for your portable devices for when you’re low on juice. And an Energon Cube would look great as such a device. If I recall correctly, in the original cartoon these cubes were filled with a hose attached to the side, so a cord or two plugged into this charger would have that same appearance.

The cube would also have a few LED lights embedded, so it could function as a portable light too. Taking it a step further, the brightness of the cube could indicate how much power it is packing. Not shown: a plug that folds out from the bottom so you can charge it in an outlet.

Update: You could apply the same idea for the Tasseract Cube seen in The Avengers.