Use a drywall square to draw two capital "T" shapes on a sheet of 3/4-inch, cabinet-grade plywood, or medium density fibercore (MDF). Make the top of the "T" 24 inches wide and 7 and a half inches tall. Draw the leg of the "T" 20 inches high---from the top of the "T" to the bottom---and 7 and a half inches wide. Center it in the top bar of the "T" shape.
Draw a straight line between the bottom corner of the top cross piece and the outside corner of the bottom of the "T" on each side, to form a vaguely triangular shape with flat ends at the bottom and each side of the top. Do this for both "T"s.
Cut along the flat ends and the diagonal lines, forming the "triangle" with a circular saw set to a depth of 7/8, for the cleanest cut.
Cut three pieces of 2 X 8 lumber, 24-inches long. Choose straight lumber with square edges, free of cracks and splintering. Glue and nail the 2 X 8s between the two "triangle" ends, one aligned with each flat end at the top sides and bottom of the "T" shape you drew initially. Nail through the plywood into the ends of the 2 X 8s. Add two 3-inch treated deck screws into each end of each 2 X 8 for reinforcement. Drive the screws in until the heads are slightly counter-sunk.
Fill the nail and screw heads with solvent-based wood filler. Allow the filler to harden. Sand the entire frame with 100-grit sandpaper. Make a second sanding pass with 150-grit sandpaper.
Apply two coats of stain and finish with a soft bristle brush, working in long, straight, overlapping strokes in the direction of the grain. Spread the finish as evenly as possible to prevent runs and drips. Allow the manufacturer's recommended time between coats.
Allow the finish to cure for 24 hours. Set the frame with the top of the "T" flat on the ground and the leg upright. Position the saddle on the top of the 2 X 8, which is now running horizontally along the top of the frame. Lift the stirrups up on top of the saddle, if the saddle has stirrups, and drive one-inch, pan-head, self-tapping, sheet-metal screws through the leather flap, under the pad of the saddle, at the front and back. Use at least four screws on each side.