If you want to model a box that is for example 20 units wide at the bottom, 15 units wide at the top, and 20 units high, here is a simple way:
1. draw a rectangle
2. extrude - when AutoCAD asks for taper angle type in the following:
(/ (* 180 (atan (/ 5.0 20.0))) pi)
where 5.0 is difference between bottom and top width (20-15), and 20.0 is the height.