How to outline BoundingBox visual queue?

There is nothing built in. You either have to build a box with traingles and draw with wireframe (and see the diagonals too) or draw a line list that shows the outline of the box.

For more flexibility, you could make a class that draws a 1/1/1 cube using lines, and when drawing it, apply a Scale Matrix based on the values of the bounding box. This way, you only have to create a single box object in memory and use it as much as you want.