Set your drums up as the drummer normally uses them, including the stool or throne, on top of a sheet of 3/4-inch-thick plywood. Draw a rectangle around the drum set, allowing enough room for the drummer to mount and dismount the platform safely. Cut it out with a circular saw. Cut two pieces of 2-by-8-inch lumber to the same length as the long edge of the plywood top. Cut three pieces of the lumber to 3 inches less than the length of the short edge.
Stand all of the lumber on its long edge and position the long pieces parallel, as far apart as the length of the short pieces. Place one short piece at one end of the long pieces and a second at the opposite end; place the third in the center. Drive three deck screws through the long side pieces into the end of the short pieces at each corner. Do the same through the center of each long edge into the ends of the middle short piece.
Place the plywood on top of this frame with the edges flush. Attach it to the top edge of the 2-by-8-inch lumber with the deck screws, placing a screw every 8 to 10 inches. Flip the riser over. Using the utility knife, cut pieces of the foam mattress topper to fit inside each half of the riser frame against the bottom of the plywood. Staple them to the plywood. Cut pieces of scrap carpet to fit on the outer sides and the top of platform; staple them in place to complete the riser.