Position the fence on the table saw so it is 24 inches away from the blade. Angle the blade at a 45-degree angle towards the fence. Run the 24-by-24-inch MDF panel through the blade on each side so that you end up with four beveled edges that are all on the same side. This is the top to your amplifier stand.
Run the 12-by-24-inch MDF panels through the table saw twice to bevel the 12-inch edges on the same side. Position the fence so it is 12 inches away from the blade and bevel the 24-inch edges of the 12-inch panels.
Position the four 12-by-24-inch panels on their 24-inch edges so they make a 24-by-24-inch square frame. Place the beveled edges that you cut on the table saw against each other so you have a right angle at each corner. This creates an even seam at each corner of the 24-by-24-inch frame. Position an 11-inch corner bracket over each corner so the ends of the bracket are each one inch away from the same set of edges on the frame. The set of edges that they are away from will be the edges that the top piece fits onto. Screw the corner brackets to the panels using eight screws for each bracket.
Set the top of your amplifier stand on top of the frame so the corners are flush. Position the 22-inch corner brackets over each corner so they are centered, one inch away from the corners of the stand. Screw the brackets to the stand using eight screws for each bracket.
Stick the adhesive furniture pads to the bottom corners of your amplifier stand if you are using your stand on any non-carpeted surface.