Here's a how-to for the Slimline Groundbreaker Coffin I posted a while back. This coffin takes up less space than most full size groundbreakers.
GROUNDBREAKER COFFIN (SLIMLINE)
1/2 or 1/4" Plywood (About 4 square ft.)
1 X 2" wood (About 5 ft.)
1" Wood Screws
(2) "L" Brackets
Small wood screws
Decorative Moulding (Optional)
Flat Moulding (Optional)
Stain or Paint
Using a square section of plywood, measure 15" along the bottom edge. Then measure 6" up the side. Use a straightedge to connect these two points. Saw along this line to create a triangular shaped piece which will be one side of the coffin. Repeat these steps to make another side piece. Refer to these as A-1 and A-2.
STEP 2: Cut a section of 1X2" wood measuring 12+1/2" long. On one side of it measure in toward the center 1/2" from each end. Draw a straight line from this point to the end corners on the opposite side of the piece. Use a saw to slice off both ends at this angle. Try to keep the saw as straight (vertical) as possible. Repeat these steps for another piece just like it. These two pieces will give the coffin sides the proper outward angle. Refer to these as B-1 and B-2.
Place one of the "B" parts FLAT on the table with the wide 12+1/2" side facing you. Stand the A-1 piece upright against the LEFT side angle of B-1. Slide A-1 forward until the back edge of B-1 is flush with the bottom and back edge of A-1. Drill a 1/8" hole thru both parts. Use a 1" wood screw to secure A-1 and B-1 together. Then stand A-2 upright against the RIGHT side of B-1 and position it the same as before. Drill another 1/8" hole thru both pieces and use a 1" screw to attach A-2 to B-1.
STEP 4: Part B-2 will be installed somewhat similar to B-1, but near the top of A-1 and A-2. Place B-2 in between A-1 and A-2 near the top, with the wide side facing you (as before). Position B-2 near the back edge of A-1 and A-2 , but angle the front side (long side) downwards a bit until the back edge of B-2 (shorter side) is STRAIGHT and VERTICAL, matching the back edge of B-1. This will give us two flat surfaces to attach the back panel to.
STEP 5: Cut a plywood piece measuring 12+ 1/8" long and 6+1/4" high. This will be the back (rear) panel for the coffin. Grasp the unit as it is built so far and flip it over on the opposite side, laying it flat. This is the actual position the prop will be displayed. (Notice the angle of the rear of the unit. It's profile now looks more like an actual coffin poking out of the ground.) Check the fit of the back panel you just cut and attach it to the rear sides of B-1 and B-2 with four 1" wood screws (two screws in each "B" piece).
STEP 6: I used the same plywood for my lid, but wanted the illusion of a thicker lid for the coffin. I did this by adding some decorative moulding to the sides of the lid. To make a lid that will fit your groundbreaker turn the coffin upside down on top of your plywood. Trace the shape of your coffin with a pencil. Cut out this shape you marked.
STEP 7: Now would be a good time to paint or stain the sides of the coffin, the lid and any decorative moulding pieces you plan on using on the groundbreaker. I stained my coffin a dark color and the moulding a lighter (honey) color. (See picture below for an example of what the prop looks like using stain.)
STEP 8: Turn the unit back over. Cut three sections of 1X2" wood (or scrap wood) measuring about 6 or 7" long. Mount an "L" bracket on the inside of A-1, with the top of the bracket even with the top edge of A-1. Repeat for the A-2 piece. Mount one of the 1X2" pieces on the bottom of the lid, against the left side. The decorative moulding will be mounted to the side of these 1X2" pieces. Repeat steps to another 1X2 on the right side bottom of lid. Add final 1X2 to the underside of the lid at the TOP. Cut the moulding for the sides and top of the lid and use the small wood screws to attach.
FINAL ASSEMBLY: I also used some flat moulding on the sides of both "A" pieces. (See pic.) Now is the time to attach those pieces to A-1 and A-2.
Place lid on top of coffin. Turn unit over and use two 1" screws to secure it to both "L" brackets.