Suggested tool: Mirror shape/object

If you later want to move the work off-center or rotate it, you must mirror the objects manually.
To mirror horizontally (left/right), duplicate the object and reverse the positive and negative values of Position X, Rotate Y, and Rotate Z.
To mirror vertically (up/down), do this on Position Y, Rotate X, and Rotate Z.

I also think it would be very useful to have a mirror duplication command or the ability to "expand" a mirror instance.