Cut the sheet of plywood into a shape that is circular on one end and square on the other, similar to a filled in letter "U." The piece should be 2 feet wide at its square end, and its sides should each be one foot long before the semi-circle begins.
Sand the edges of the wood to remove any splinters.
Cut two, one-foot lengths of the 2 x 4 board. Fasten them with bolts perpendicularly to the plywood, two inches from either end side, after drilling corresponding holes. Ensure they hang over the square end of the plywood by three inches. If you consider the plywood to be a filled-in letter "U," the 2 x 4 pieces will run along the parallel sides of the "U."
Cut a piece of the 2 x 4 to fit perpendicular between the two one-foot lengths of 2 x 4, and fasten it to the 2 x 4 pieces and plywood with bolts. This piece will serve as a stabilizer.
Drill a hole through the three-inch overhang of the two one-foot lengths of each 2 x 4, and measure and drill corresponding holes in a piece of the wall framing in a garage or unfinished basement.
Fasten the speed bag mount to the frame of the wall using bolts and nuts.
Screw the speed bag mount into the center of the piece of plywood, and loop the speed bag through the metal ring.