Purchase “A-grade” 1-by-6-inch dimensional pine lumber. You will need 38 board feet, or five 8-foot boards, to build one 4-by-8-foot platform that stands on six 18-inch legs.
Balance two 8-foot boards on their edges, spaced approximately 48 inches apart.
Cut three boards 47 inches long. Mount two of these short boards inside the two long boards on the ends. Drill three pilot holes through the sides of each of the long boards where they form a corner with an end board. Use 1-inch wood screws to connect the boards at the corners and form a rectangular 4-by-8-foot frame. Glue the joints with wood glue for extra strength.
Mount the third 47-inch board inside the frame, in the center. Drill pilot holes and screw this center support board to the frame from the outside of the frame.
Lay a sheet of AC-grade, 4 foot by 8 foot by ¾ inch thick plywood on top of the frame. Test-align this plywood “lid” to the edges of the frame, flush. If the fit is good, remove the lid momentarily.
Apply a generous bead of wood glue to the edge of the frame. Lay the plywood sheet on top of this glued edge. Drive a 1-inch wood screw through one corner of the lid, connecting it to the frame. Make adjustments in the lid alignment by pivoting the plywood sheet on this first screw. Drive more 1-inch wood screws around the four sides of the plywood lid on 6-inch centers.
Examine the finished product. You have a lightweight 4-by-8-foot riser that stands 6 ¼ inches tall without the addition of “legs.” Continue if you need to increase the height of the riser.
Measure more 1-by-6-inch lumber for riser legs. If you want the riser to stand 18 inches tall, cut six legs 17 ¼ inches long. Cut six more legs 11 3/4 inches long.
Apply wood glue to the shorter of the two boards. Sandwich one of the shorter boards together with one of the longer boards. One end of the two boards should be flush. The other end will not be flush because one board is shorter than the other, forming a “step.” Screw the two boards together using 3/4-inch wood screws, in a double X pattern, to form one complete riser leg. Repeat to assemble six legs.
Turn the platform over on its lid. Insert a leg at one corner of the platform framing. The shorter side of the leg faces the outside of the platform. The taller side of the leg should fit inside the frame and against the lid. The frame will rest on top of the shorter “stepped” side of the leg board.
Screw or bolt the leg to the frame with 3/4-inch wood screws (X pattern) or 1 ¼-inch carriage bolts (two bolts per leg, offset slightly).
Repeat installing legs at the remaining three corners of the platform and both sides of the center support frame.
Flip the platform up on its legs. Adjust as necessary to level the platform on the legs. Reinforce taller legs with cross bracing as necessary.