We used green treat 4x4's for the posts and cedar 2x6's for the main box construction. This box was made to match our existing 4x8' box, but we wanted this to be 4x4' instead. Eventually, we'll have 3 boxes total, but we're taking it in steps.
We wanted the front of the box to be roughly 11 inches tall to match the height of our existing box. Because the area in which we installed the box is on a slight slope, 3 sides of the box (the back and sides) had to be 6 inches deeper. We cut eleven 4-ft long cedar planks and the posts were cut to approximately 32" long with a slight angle. The angle, we found later, to be absolutely useless, since the soil was so rocky and hard that pounding it in with the point was impossible.
We used decking screws to put it together since they don't rust, making sure to pre-drill holes. It doesn't really take two people, but it sure does make it go faster. Then came the hard part: Installation.
Then we (Mark) marked around the box with a spade.
After removing the grass from the area, Mark dug holes for the posts. I dug down the sides while he was digging the holes, since some of the sides would be buried slightly since the area sloped.
Here you can see how the terrain slopes and how we had to make one side of the boxes deeper than the other.