Creating a cube, cube, rectangle, or prism in Voxler can be straightforward with the use of the Inventor language. Attached is a sample IV file. Refer to the Inventor Mentor for the programming manual for the Inventor language. I've found that searching the web for Inventor IV files is a great way to find examples.
You can also use the PLY format to draw a cube. I've also attached a sample PLY file.
Updated October 7, 2016