Draw a full-sized template on the shelf paper. Tape several sheets together if necessary. Start by drawing a long cross. The cross' bar should intersect the beam at 58 1/2", leaving 17" inches at the top. The bar itself should measure 17" on each side. Use a carpenter's square to make the cross perpendicular. Draw a 2' line perpendicular to the top of the cross and a 17" line across the bottom. Connect the endpoints at the top and bottom of the template to the bar of the cross. You will use this template when measuring and cutting the side panels.
Draw a template on shelf paper for the base of the coffin. Repeat the process you used for the first template, with smaller dimensions. The beam's measurements should be 57 3/4" and 16 1/4"; the bar should measure 16 1/4" on each side. The top and bottom lines should measure 22 1/2" and 15 1/2".
Measure and cut the wood. Cut the base according to its template. Measure and cut the six side panels in accordance with the first template. Each side panel should be 1' in width.
Dry-fit the side panels onto the base to make sure that they fit properly. Connect all points with wood glue, then with wood screws.
Measure and cut the coffin's lid. Do this by laying one of your plywood sheets on top of the bottom assembly. Trace the outline of the assembly onto the plywood.
Attach the piano hinge to the lid using the smaller wood screws.
Plug any remaining holes with wood filler. Coat the coffin twice with wood stain.