Cut 2 2x4"s 20 1/4" long. Now cut a 15" angle off of each one. (If you can't measure angles, don't worry keep reading)
Now measure up 15", and screw the top of the front of the chair to the leg.
If you couldn't measure the angles, just screw the board on. Then you can see how the corner sits on the ground, draw a line parallel to the ground and cut it off. Now cut the top off parallel to the ground.