This is a very easy project and can be done relatively cheap. First, a note about the measurements. 1" X 4" boards in most cases actually measure .75" X 3.5", thus the measurements are assuming the boards are 3.5" wide. If you get 4" boards, simply add .5" to each board in the measurements. Second, when picking your wood, you may want to look for boards that look old, beat up or have knots in them. This will add to the overall effect. And last, you may want to leave a little bit of a gap between the boards so that later on you can add a light or pump fog into the coffin, having it seep out of the cracks.
The first order of business is to cut 6 boards in half, and then using boards run perpendicular to hold the pieces together. You will create two sides of 6 boards each. Place one perpendicular board 10" from the top and one 12" from the bottom. Using the drawing, mark and cut the toe-pincher shape in each panel. This is done by marking 5.25" in on each side at the top and 3.5" in from each side at the bottom. Now mark 18" down from the top on each side and connect all these points to create your shape.
Build the 4 sides using 3 boards each this time. The short sides are 18.5" long and the long sides are 30.5" long. Add a perpendicular 10" down on the short side and 12" up on the long side. Make sure that you allow this board to overlap 1.5" on each side so that when you put the top and bottom together, the perpendicular boards will be flush. You will need to cut 1" X 2" strips to install on the opposite inside of each side to hold the boards together.
The top is simply 3 pieces of board 10.5" long, fastened together using 1" X 2" strips on the inside. At this point you may want to mitre all the side and top edges so that everything will go flush against the other pieces. This step isn't necessary, as we are trying to build an old and busted looking coffin. Take the sides and top and bottom and begin piecing them together using rafter brackets. These are available in the lumber section of Home Depot and come in a variety of styles and only run about 20 cents each.
Once all your pieces are together, choose the angle at which you wish to cut the bottom. This will provide the effect that the coffin is buried. In this example I cut so it leans to one side, but you may wish to cut the back shorter so that it leans backward. As a final detail, you may wish to cut a cross into the front of the coffin using a jig saw. If you do this, be sure to secure the boards behind the cross using 1" X 2" strips so that they don't become loose.
NOTE: 1" X 4" board is not always exactly 1" X 4", you most likely will find your boards to be .75" X 3.5". Be sure to measure the wood you bought before cutting anything.