[fwlr]

Faces of the cube with the same color as the background and opacity 0.2 would almost certainly work. My next thought is duplicate the original cube, remove the characters from the duplicate cube and set the color on the duplicate only, then move the duplicate cube back one pixel in the z direction.

[kilovoltaire]

Ah yeah faces with same color as background and low opacity sounds like a good approach.

Here's an editable copy of the code if anyone sees this and wants to try adding the faces - https://jsfiddle.net/t0crk5do/