Perfect Boxes in Milkshape?

How do I create a Perfectly Symmetrical Box in Milkshape?

Is there a constrained aspect ratio?

If not, can I do it in Magos' Model Editor?

Select the vertices on one side of an axis, and mirror them. Undo that, and move them accordingly, until mirroring results in the vertices going precisely on top of the ones on the other side. Make extreme use of the "snap to grid" tool (ctrl+G).

Repeat for all axes.