cutting mdf for a wedge box

This is driving me CRAZY... I've done so much calculus in college I think that all of my geometry skills are gone and I've hit a wall. I am trying to build a trapezoid shaped box out of 3/4" MDF with the following external measurements: 16" high 21" deep on the bottom 11" deep on the top 31" wide I can't figure out how to cut the wood for the angles, or how it should go together. I know that if I just cut the pieces out with those measurements, it won't work... is there an easy way to figure out how much longer or shorter I should cut each panel so that they will properly overlap and/or meet at the correct angle?

Well im either bein retarted or this should work.. The side piece measurements are 9.5" on the top, 19.5" on the bottom, the angled edge is 17 and 6/16", and the straight edge is 14.5" The bottom of the box piece is 31" by 20.25" The Top of the box is 31" by 9.5" The Slanted front piece is 18 and 14/16" by 31" The straight back piece is 15.25" by 31" so, this way the side pieces will be overlapped on each of their edges, the back piece will overlap the top piece but not the bottom, and the front piece will overlap both the bottom and top pieces.