AR5 Perspective Box Possible?

[javafuel]

I'm trying to construct a box perspective template with four walls and two floors. The two back walls of the box won't connect to the rest of box, so they don't move properly (become offset) with the changing in horizon line's height. Please advise.

I want to have a box perspective template that I can quickly modify the vanishing points and horizon line without needing to adjust the two back walls of the box.